Copyright © 2006-2007 Network for sustainable use of energy in water and wastewater systems
The topic has been divided into the following sections:–
The primary objectives of anaerobic digestion (AD) are solids stabilisation and reduction and biogas production to permit power generation. As it is routinely practiced, it is only partially successful at delivering these objectives. Average solids reduction can be as low as 30-35% and the level of stabilisation achieved influences the outlets available for the final product. Biogas yields can also be similarly less than optimum.
The current drivers for considering adoption of a pretreatment stage in sludge management to improve the digestion performance are:
The vast majority of AD facilities in the UK and the rest of Europe are mesophilic and a considerable amount of research effort has been expended over the last 15 years to try to increase the effectiveness of these facilities. This effort has focused on the overcoming the rate determining step in the AD process, that is, cell hydrolysis. Anaerobic digestion comprises four principle stages, hydrolysis, acidogenesis and acetogenesis, together referred to as fermentation, and methanogenesis. It is the hydrolysis stage ie the biological degradation (solubilisation) of cellular organic matter, that is the principal rate determining step and which is the target of pretreatment processes. The degradation or lysis of sludge cells releases the cell contents and makes them available to biological breakdown. In addition, facultative anaerobic micro-organisms, which otherwise can partly survive the digestion process and can be responsible for the residual content of organic matter in the digested sludge, are made accessible to increased degradation. Through the utilisation of the proteins and polysaccharides contained in the cell interior as carbon source the C/N ratio can be improved. Denitrification, if present, can thus be supported and enhanced by using this carbon source. There is a further advantage in the subsequent digestion process. The cell lysate on the one hand stimulates the function and the growth of the microorganisms, and on the other hand, it contains a series of enzymes which are directly necessary for the degradation of organic matter. Enhanced degradation can also be achieved by treating the anaerobic sludge in the heating and recirculation system of the digester or in between two digesters operated in series so a pretreatment stage should not be assumed to be the appropriate approach for a particular works.
Methanogenisis can also be a rate determining stage in the digestion process since methanogenic bacteria are much slower growing and more fastidious in their environmental requirements than are hydrolytic or acidogenic bacteria. Hence, the rate of digester start up and response to shock loading is primarily controlled by the development of the methongenic bacterial population.
A wide range of pretreatment techniques have been tested at a bench scale, and increasingly these are being scaled up to pilot plants located on operational WWTWs. A small number have become commercially available (Sections 10 & 11) Techniques divide into four broad categories; mechanical, thermal, chemical and biological. The primary objective of all pretreatments regardless of method is the destruction of sludge flocs and the rupturing of cell walls to release cell components so that they are accessible for chemical and biological processes. Their sub-division by mode of action is to some extent arbitrary since for example, steam explosion could easily be categorised as a thermal process rather than mechanical and freeze/thaw as mechanical rather than thermal. Likewise, ultrasonic treatment causes some simultaneous heating and both thermal and irradiation effects have been observed with the application of microwave.
Potential benefits which have been claimed for pretreatment stages are:
Potential disadvantages of pretreatment include:
Pretreatment methods may be used individually or in combination to deliver optimal results for a number of objectives. However, it should be noted that the objectives of pretreatment need to be clearly defined before a technology is selected, since a treatment to improve, say degradability, may have negative impacts on another treatment stage, for example, dewatering. With regard to dewatering, in particular, pretreatment has the potential to both increase the ease of dewatering through the breaking of floc structure and releasing of floc held water, and decrease the ease of dewatering by increasing the degree of sludge solids dispersal and consequently increasing coagulant requirement.
Scum, foaming and sludge bulking cause operational difficulties in sludge digesters. Bulking is defined as poor settling characteristics in sedimentation tanks and waste activated sludge (WAS) is particularly susceptible. The main cause of sludge bulking is the growth filamentous bacteria and algae in long strands that clog up the process. When favourable growing conditions are present these filamentous organisms predominate and with their high surfaces area they use most of the oxygen in the tank and form a low settleable sludge. Pretreatment techniques designed to disrupt cell structure are effective at destroying these filamentous bacteria, so in some circumstance pretreatment can improve dewatering.
Degradation processes may lead to increased concentration of organic compounds in the sludge liquor ie increasing its strength and consequently it load on the works when returned for further treatment. The increase in nitrogen is caused by the enhanced degree of biodegradation and is not a direct consequence of disintegration. However, the increase in flocculant demand is a direct result of disintegration, because the increased number of small particles needs more polymers to neutralise the surface charge (Kopp et al., 1997; Müller, 2000)
The choice of pretreatment is also influenced by sludge type. For example, enzyme treatment is particularly effective for sludges with high lignocellulose content and primary sludges (Müller 2001).Their use in secondary sludges (waste/excess sludge) can result in the degradation of the enzymes themselves before enzymic hydrolysis commences. That said, primary sludges are predominantly composed of easily degraded components hence, pretreatment is of principle benefit for secondary sludges, since it is the resistance of microbial cell walls to degradation which is responsible for the long retention times required for biological stabilisation.
Extracellular polymeric substances (EPS) are major components in activated sludge flocs and comprise primarily of a matrix of carbohydrates, proteins ,including enzyme) and humic substances and, to a lesser extent, also include lipids, uronic and deoxyribonucleic acids. Interactions between EPS, multivalent cations, hydrophobic interactions and hydrogen bonds give rise to the formation of a network of polymeric substances in activated sludge (Wawrzynczyk et al., 2007, Beijer 2008)
EPS originate from active secretions of bacteria and from the organic and inorganic debris present in the activated sludge. The formation of EPS depends on a variety of functions and the composition and quantity of the EPS therefore vary markedly between sludges. Some of the factors affecting the composition and quantity of the EPS are the type and age of the sludge, the types of microorganisms present in the flocs and the cations available. (Chrysi et al 2002, Wawrzynczyk 2007, Beijer 2008). The nature and variety of EPS are a substantial cause of the variability in the response of different sludges to pretreatment techniques.
The methods used to determine the improvement in digestion achieved by pretreatment are not universally agreed, in part because the particular objective of conducting the pretreatment will dictate what is an appropriate measure of performance. Also depending on the manner in which the tests are conducted, some measurements can be taken more easily and more accurately than others. Reliable sampling at a WWTW for example can be particularly problematic.
Widely used measures are of chemical parameters: increment in soluble chemical oxygen demand (SCOD); increment in soluble protein concentration (SPC) and in filterable volatile solids concentration (FVS); reduction in volatile solids and biogas production. Climent et al (2007) used the ratio of filterable volatile solids to total volatile solids in treated and untreated sludge as an indicator for the comparison of thermal, microwave and ultrasonic pretreatments. (Climent, et al 2007) etc. However, they found that an improvement in the FVS/TVS ratio delivered by all three treatment types did not generally reflect an improvement in biogas production. Climent et al (2007) were working with thermophilic digestion rather than mesophilic, suggesting that the benefits to be gained from pretreatment are less marked for thermophilic digestion where biogas yields and sludge stabilisation tend to be higher routinely.
Increased soluble COD has also been found not to reliably reflect an increase in biogas production (Delgenčs et al 2000). Applying a thermochemical pretreatment of sodium hydroxide induced a pH increase to 12 and a temperature of 140°C for 30 minutes was found to deliver a 70% increase in soluble COD but no improvement in the biodegradability of the sludge. Gas yield was not measured in this experiment (Delgenčs et al 2000).
Gas yield is difficult to measure in laboratory tests as it involves the setting up of additional equipment to undertake a digestion stage, which is time consuming and takes up a lot of space. It can also be difficult to ensure complete capture of the gas. Hence, there has been a tendency to try to use other parameters as indicators of the effectiveness of a pretreatment. However, it will be seen in the discussion of the specific pretreatment methods below, that none of the chemical parameters which indicate that cell degradation has taken place can be said to reliably reflect a s subsequent increase in biogas yield, though in some studies the correlation has been very good.
Full scale testing of technology can provide good data on gas yield since the equipment for monitoring gas production is already in place at STWs. However, the cost of full scale testing is considerably higher due to the scale of equipment required and the time necessary to ensure stable digester operation. Zábranská et al., (2006) suggest that for accurate assessment of the effects of sludge disintegration, it is necessary to monitor gas production for 12 months before the introduction of a pretreatment and at least six months after it has been installed.
One method of comparing treatments methods would be to consider biogas yield per unit of energy employed in the pretreatment process. This would allow decisions on the most appropriate technique to consider the net energy cost or benefit of introducing a pretreatment technology. However, other non-biogas related benefits can be derived from introduction of a pretreatment stage to a specific STW. Asset capacity, availability of sludge outlets and operating costs all need to be considered when determining if pretreatment is beneficial to the operation of a specific works. In other words, there may be little or no benefit from direct energy output from increased biogas yield but when the whole system is considered there may be a net reduction in energy use, eg from reduced dewatering or transport costs for sludge.
Mechanical methods of pretreatment include the following techniques:
The manner in which all mechanical treatments work is essentially the same, cell walls are ruptured and flocs disrupted by the application of force.
Most pretreatments are, as the name suggests, applied to sludge before it enters the digester. However, Muller et al., (2007) tested a method which applies mechanical shear to actively digesting sludge. The stated advantages of this are that rather than applying shear to the whole volume of sludge, much of which would be readily digestible without pretreatment, shearing is applied only the recalcitrant portion, hence less energy is expended. Post digestion flocs are weakened by the elevated temperatures in the digester and hence more susceptible to shear, and similarly digested sludge is more sensitive to shear than raw activated sludge (Muller et al., 2007). However, potential down sides to this approach have also been identified including the potential disruption of the digestion process by the disassociation of methangenic and acetogenic bacteria such that their symbiotic degradation of sludge solids is disturbed. That is to say, the process of shearing within the digestion process will affect the useful microbial population as well as the greater mass of the sludge matrix. No negative effect on digester performance was detected during this study, possibly due to the low proportion of the sludge which had undergone shearing that was in the digester over the sludge retention period (Muller et al., 2007).
The Muller team tested the response of four different sludges to the imposition of mechanical shear on sludge recycled to the input side of the digester, using reduction in volatile solids as the measure of benefit. Only one demonstrated there to be any benefit, with a doubling of VS reduction following shearing. However, further testing of this sludge at a full scale within the treatment works showed that the shear method delivered an increase in the colloidal size fraction of the sludge, greater degradation of proteinaceous materials and associated ammoniacal –N release, a reduced total and volatile solids concentration and a 46% increase in the biogas yield, though no reduction in total or faecal coliforms. This is a useful demonstration of the benefit of pretesting of sludge to identify a pretreatment method which delivers sufficient benefit to be worth installation.
The increase in the percentage of colloidal size fraction within the sludge solids has the potential to increased the polymer requirement for dewatering. Indeed, an 84% increase in polymer demand was detected during the full scale testing by Muller et al., (2007), the cost of which would probably negate any benefits gained from increased biogas yield and decreased sludge solids. However, they were operating in a double digester system where the second digester was unheated and used for settling and thickening the sludge. After this secondary digestion the overall polymer of the works are actually reduced. The reason for this was not determined but they suggest that shearing combined with a secondary settling and thickening process prior to dewatering may be the optimum combination of processes.
The combining of waste activated sludge disintegration with a thickening stage demands very low energy input relative to the benefits gained (Zabranska et al, 2006). The partial destruction of excess activated sludge cells can be achieved during sludge thickening by means of a centrifuge equipped with a disintegrating device. The disintegrating gear is mounted at the end of the flow of thickened sludge and does not influence the centrate quality (Dohányos et al., 1997). The method is capable of producing a 10–15% degree of disintegration (Müller et al., 2004), while also being capable of handling high volume flow rates. Energy consumption is dependent on the centrifuge output and, for high capacity centrifuges, is almost negligible (Zábranská et al., 2000).
Zábranská et al 2006 looked at a the full-scale operation of lysate thickening centrifuges across a range of difference sizes of STWs. Biogas yield was increased by between 15 and 26%, the extent of the increase being influenced by activated sludge age, content and type of the organic material in mixed raw sludge and the hydraulic retention time in digesters. The greatest increases in degradation were achieved with younger activated sludge, shorter retention time in digesters and with a higher activity of anaerobic microorganisms in digesting sludge.
A secondary benefit obtained from using a thickening centrifuge is a considerable decrease in sludge viscosity. The limit for pumping of thickened excess sludge is typically around 6% TS. Greater thickening causes problems with clogging especially when pumping over longer distances. Disintegration enabled the excess activated sludge to be thickened to 9–11% TS, thereby reducing the volume of the input sludge, increasing the retention time of organic matter in the digester and decreasing the energy demand for digester heating. Hence, in addition to increased energy production from enhanced biogas output, energy demands in other component stages of the treatment process were reduced (Zábranská et al 2006). The Zábranská et al.,(2006) studies were conducted over a two year period at three different treatment works of varying size. Eskicioglu et al., (2007a) have suggested that reduced viscosity in sludges improves mixing which will itself enhance biogas production.
As with every aspect of sludge treatment the outcomes achieved from a specific technology vary with the sludge characteristics and if possible testing in a pilot plant is advisable before installation of a full-scale equipment.
Pulse power technology uses electrical pulses in the megawatt range with a pulse duration of less than 10 milliseconds to induce shock waves in solid and liquid media. Two electrodes are immersed into a liquid with a low conductivity and the material to be treated is placed at a short distance from the electrodes. A voltage of several 1000 V is passed across the electrodes until there is a ´breakthrough´ between the electrodes and the resulting current creates a shock wave, which is transferred by the liquid to the material to be treated. Because of the high inertia of the organic substances in the sludge, very short pulses with a high amplitude and frequency are employed. A pressurised air supply of about 44 Nm3/min. at a pressure of ~7,5 bar on site are required for generating the nitrogen.
The pulse reactor can be fitted directly into the sludge feed line to the digester. The space between the electrodes is filled with nitrogen to achieve an electrically stable discharge. A standard 230-240 V 50Hz power supply is sufficient for the pulse generators for several pulse reactors and a nitrogen sieve to produce nitrogen from pressurised air.
The system has only been used on a pilot scale – 36m3 hr-1 for one pulse reactor – at a WWTW with a capacity of 125,000pe comprising three digesters with retention time of 20 days. The pulse system was installed on the feed line to one of the three digesters and achieved a 12% increase in biogas production. System optimisation such as increased voltage would be expected in improve this to around 20% additional biogas production. The technology has been developed by Technical University at Braunschweig in co-operation with TZN (Technologiezentrum Nord) GmbH and an American partner, Scientific Utilization (Stowa 2006).
Ultrasound is the name given to acoustic phenomena that are beyond human hearing, both low and high frequency, though it is generally quoted as being frequencies that are above 20 kHz (Tiehm, et al 1997). The distinction needs to be made between low frequency, high power and high frequency, low power ultrasound. High frequency ultrasound produces no permanent effect and is used in medicine. Low frequency ultrasound, between 20-400 kHz results in the formation and collapse of bubbles or voids, a process called cavitation, which can cause significant physical and chemical changes in a liquid (Portenlanger 1999). Cavitational collapse produces intense local heating and high pressure on liquid–gas interface, turbulence and high shearing phenomena in the liquid phase. Because of the extreme local conditions, OH, HO2, H radicals and hydrogen peroxide can be formed. Thus, the effective mechanism of ultrasound is a combination of different phenomena: chemical reactions using radicals, pyrolysis, combustion and shearing (Bougrier et al 2005).
The mechanisms of ultrasonic treatment are influenced by the energy supplied, ultrasonic frequency and the nature of the sludge treated. Cell disintegration is proportional to supplied energy (Bougrier et al 2005, Cui and Jahng 2006). High frequencies promote oxidation by radicals, whereas low frequencies promote mechanical and physical phenomena like pressure waves. With complex influents, radical performance decreases. The process has been shown to be most disruptive at frequencies below 40 kHz and high intensities (Mason 1990, Tiehm et al 2001, Bougrier et al 2005).
Ultrasound is an attractive pretreatment technology because it does not use any chemicals, had a low energy demand, and is relatively easy and inexpensive to install. The reduction in sludge retention times that have been demonstrated by various studies also suggest that treatment facilities which are under pressure in terms of capacity may benefit from the introduction of ultrasonic pretreatment (Bougrier et al 2005).
Many studies have been conducted using ultrasound, also referred to as sonication, as a pretreatment for sludge. However, the terms used to describe the method of treatment vary, eg kJ/kgTS or W/ml/unit time, and it is not always possible to directly compare results to determine the most appropriate treatment conditions.
Bench-scale studies have often shown there to be increased VS destruction and biogas yield but at an operational scale these finding are frequently not reliably reproducible, the main difficulty being ensuring that the sound waves penetrate the full bulk of the sludge to produce sufficient cavitation (Brennan et al 2008). The research studies are generally conducted on very small volumes of sludge, typically 0.5 to 3l which can be thoroughly sonicated.
Hydrolysis is a rather slow process catalysed by extracellular enzymes such as proteinases, amylases, lipases and nucleases. Mao and Show (2007) found ultrasound treatment to effectively pre-hydrolyse the sludge, increasing the soluble COD content by between 48 and 161% as sonication intensity increased. Subsequent COD removal during a 16 day digestion period was improved by 54, 61 and 67% for each of the three sonication treatments. Particle size and total surface area were increased by the ultrasound treatments suggesting that the rate of hydrolysis was improved by a greater availability of enzyme adsorption sites. Likewise, Bougrier et al (2005) found the proportion of soluble solids to be increased in proportion to the specific energy delivered to the sludge (See figure below).
Gas yield increases of between 15 and 48% and averaging 22.4% were reported by Brennan et al (2008) from a four month study of operational mesophilic digesters handling mixed primary and activated sludges, 60 and 40% respectively. The ultrasound was delivered using a Sonix ultrasound containerised plant, supplied by Purac Ltd. This was located on the SAS pipeline diverted through the container. The horns were of radial design, the residence time within the active zone of the reactor was 2 seconds and the ultrasound frequency was set at 20 kHz.
Considerable operational difficulties with the digesters were experienced during the period of the trial which almost certainly contributed to the variability in gas yield over the test period. Solids reduction was not significantly increased by sonication and neither was VS destruction, which is somewhat odd given the increased gas yield. This inconsistency between increased biogas yield and increased VS destruction is not uncommon in studies of ultrasound pretreatment.
Mao and Show (2006, 2007) studied the effects of ultrasound specifically on the three digestion phases of hydrolysis, acidogenic and methanogenesis to determine the specific effects on each of the three digestion phases. Activated sludge was sonicated at a low frequency of 20 kHz and different sonication densities or energy intensities, namely 0.18, 0.33 and 0.52 W/ml for one minute using a probe transducer.
Both biogas yield and quality were improved by the ultrasound treatments use by Mao and Show (2007). Methane concentration increased from 57% in the control digester to 61, 64 and 69% in the different ultrasound treatments. These enhanced levels are in line with results achieved in a study by Tiehm et al (2001) in which methane concentration in the biogas was increased by between 61 and 69% following ultrasound treatment. Biogas yield increased by between 19 and 52% in the Mao and Show (2007) study. Tiehm et al (2001) report enhanced gas production by ultrasound treatment ranging from 16 to 122% in 22 days while Wang et al (1999) report increases of between 12 and 69% in 11 days digestion. While the Mao and Show study suggest that ultrasound treatment can provide substantial benefits to digestion, it should be noted that their study was conducted at a bench scale using 1.5l digestion vessels.
The use of ultrasound as a sludge pretreatment is one of the closest technologies to full commercial development. Unlike many of the techniques discussed here studies are consistent in reporting an increase in biogas yield following its application.
Thermal disintegration processes have a higher energy consumption than mechanical methods, but they can potentially use waste thermal energy instead of electrical energy. The input of thermal energy is generally achieved using heat exchangers or by application of steam to the sludge. Excess heat obtained within the STW can be used, thus reducing the energy cost of pretreatment significantly.
Thermal pretreatment has been studied using a wide range of temperatures ranging from 60 to 270°C. Temperatures over 200°C have been found to result in the formation of refractory compounds. The most common treatment temperatures are between 60 and 180°C. Treatments applied at temperatures under 100o C are considered as low temperature thermal treatments (Climent et al 2007). There is evidence that thermal treatment around the 70°C level enhances the activity of the bacterial population, improving the degree of digestion, but this is not thermal hydrolysis since the cell walls are not disrupted by the action of heat, rather they are acted upon more effectively by biological hydrolysis (Climent et al 2007). Hence, thermal treatment at the lower end of the temperature range should be regarded as pre-digestion or enhanced digestion rather than thermal pretreatment (Climent et al 2007).
Thermal and thermochemical pretreatments were tested on two activated sludges by Bougrier et al (2006). Soluble COD was increase from 3% and 7% in the sludges before thermal treatment at 170°C from 30 minutes to 57% and 49% respectively afterwards. One sludge was also treated with potassium hydroxide to a pH of 10 and 130°C led to a similar rate of solubilisation as 170°C (Bougrier et al 2006).
The same study also looked at comparing the effect of different pretreatment temperatures on the results of the subsequent anaerobic digestion. Pretreatment temperatures of 150 and 170°C resulted in sludge solids reduction increasing from 25% during digestion of the untreated sludge to 43 and 52% for the thermally treated sludge (Bougrier et al 2006). Likewise COD removal was increased from 34% to 6 and 70% respectively. Bougrier et al (2006) also found the methane yield to to have been increased by thermal pretreatment - 132 L kg-1 CODin to 217 and 234 L kg-1CODin. However, the additional energy required to achieve the higher temperatures meant that there was no net energy gain by the increase methane production compared with mesophilic operations.
The optimal temperature treatment is often reported to be around 170°C: Li and Noike (1992) 170°C, Haug et al. (1978) 175°C, Sawayama et al. (1995) 175°C, Kepp et al. (2000) 165°C as reported in Bougrier et al (2006). Generally, up to these temperatures the proportion of SCOD increases but at higher temperatures no benefit is obtained. It might be expected that at lower temperatures longer contact time is required but this is very sludge specific as can be seen in the table below.
The application of thermal hydrolysis is probably the mostly widely adopted and most mature technology operationally.
Microwaves are an electromagnetic radiation with an oscillation frequency of 0.3–300 GHz and wavelengths of 1cm to 1m respectively (Vollmer 2004). Heating applications generally use a frequency of 2450MHz with a wavelength of 12.24cm to avoid interference with telecommunications and cellular phone frequencies. Frequencies around 900 MHz, which can provide up to 100kW and longer wavelengths (37.24 cm), are used for larger process heating applications where deeper penetration into material is necessary. However, if microwave is used for heating or drying a product in thin layers with a large surface area, shorter wavelengths (2450 MHz–12.24 cm) are adequate ((Eskicioglu et al., 2007a).
Industrial applications of microwave heating for chemical reactions in place of conventional heating are becoming increasingly popular due to the dramatically shorter heating times and lower energy use achieved using microwave technology. There is also a growing interest in the large scale use of microwave in wastewater treatment for the same reasons. There have been many bench scale studies of the effect of microwave or irradiation on sludge destruction and biogas yield. Park et al., (2004) found that, at a maximum, rates of COD removal and methane production in microwave pretreated secondary sludge to be 64% and 79% higher than those of the untreated control. Pino Jelcic et al., (2006) similarly found 17% and 6% increases in biogas yield in microwave irradiated WAS compared with untreated and conventionally heated controls. This study also found enhanced dewaterability and pathogen reduction with microwave treatment compared with conventional heating.
Mixing during microwave treatment is required to ensure the full volume of sludge is treated since aqueous systems subjected to microwave irradiation inevitably undergo poor heating of the internal volume in large samples because of high microwave absorption by the outer layers (Whittaker and Mingos, 1994). In-line microwave treatment during sludge pumping may be one was to minimise this effect.
The chemical mechanism of microwave interaction with materials, nor the microbial destruction (sterilisation) mechanism of microwaves in biological systems is fully understood, despite the considerable research into microwave treatment (Eskicioglu et al., 2007a). The evidence appears contradictory, with some studies concluding that the sole effect is thermal, there being no difference between the effects of microwave and those of convention heating to the same temperatures, while others suggest that cell damage is greater where microwave has been used and that cell lysis occurs as a result of athermal effects such as the action of the electromagnetic field on molecular bonds (Eskicioglu et al., 2007a).
One difficulty with comparing the thermal and athermal effects of microwave is the longer exposure time necessary for conventional heating to reach the same temperature as microwave heating. Another is the availability of equipment for measuring time-temperature profiles that do not interfere with the action of the microwaves.
An intensive study by Eskicioglu et al., (2007a) was designed to eliminate the difference in temperature profiles in order to establish if the athermal effects of microwave action discussed in the literature, were real. Using tightly controlled conditions, they determined that where the heating profile and final temperatures were the same, the ratio of soluble COD (SCOD) to total COD (TCOD) achieved using conventional heating and microwave heating were the same and hence that there was negligible athermal effect of microwave of the solubilisation of WAS. However, they did find that the rate of heating affected the SCOD/TCOD ratio, a slower rate of heating to the same end point increasing the proportion of SCOD present by 4-6%. They also found microwave to be more effective at N release from cellular material, though at higher temperatures, 70 and 96 compared with 50°C, both treatment types reduced ammonia-N, protein and sugar concentration.
In summary, microwave pretreatment of sludge prior to anaerobic digestion has been found to increase biogas production, reduce sludge viscosity, improve dewaterability, and improve pathogen kill over both untreated controls and those pretreated through conventional heating. Microwave treatment also have the potential to be more cost effective than conventional heating since it uses much less energy. Unfortunately that energy is electricity and thermal techniques can employ waste heat sources on STWs, so the balance of energy cost is not necessarily wholly in favour of microwave irradiation. No evidence was found of the operational use of microwave pretreatment of sludge.
The effects of strong acids and bases on solids solubilisation, VS destruction, COD and biogas production has been widely studied, though there appears to be greater emphasis on the effects of bases rather than acids.
Knezevic et al. (1995) studied the effect of low-level alkaline pretreatment of WAS on enhancement of the subsequent anaerobic digestion of a mixture of primary and WAS. WAS chemical solubilisation by the addition of different dosage of Ca(OH)2 or NaOH. Results showed that both chemicals improved WAS solubilisation with NaOH being more effective for the same chemical dosage and mixing time. The digester reactors fed with WAS treated with NaOH resulted in a higher volatile mass reduction, total and soluble COD removal and gas production. Dosing with 12meq/l NaOH permitted a decrease from 25 to 10 days SRT without affecting the overall process performances and effluent quality.
Jeongsik et al (2003) studied the effects of various alkaline pretreatments to enhance the efficiency of anaerobic digestion of WAS. The use of different chemicals such as NaOH, KOH, Mg(OH)2 and Ca(OH)2 at pH 12 was investigated. The results showed that, at ambient temperature, COD solubilisation values of 39.8%, 36.6%, 10% and 15.3% were obtained by adding NAOH, KOH, Mg(OH)2 and Ca(OH)2, respectively. The relevant difference in COD solubilisation between monobasic and dibasic agents is probably due to the partial dissolution of dibasic alkaline agents.
The effects of different concentrations of NaOH were subsequently investigated. Results indicated that the rate of COD removal increased as the NaOH dosage increased with a maximum value of 43.5% when 7g/l NaOH were added. Above this concentration, a lower rate of COD solubilisation was observed. Subsequent digestion of sludge treated with 7g/l NaOH for 7 days at 37°C resulted in VS reduction in the pretreated sludge of 29.8% compared the 20.5% of the control while methane and biogas production increases of 12.7% and 13.3%, respectively were observed (Jeongsik et al. 2003 )
The benefits of pretreatment are most marked when applied to secondary sludges which have a much higher proportion of microbial cells. However, many pretreatment tests are conducted on a mixture of primary and secondary sludges. In contrast, De Franchi (2005) compared the benefits to digestion obtained from pretreating a primary sludge and a secondary sludge from the same source with different chemicals. The sludges were pretreated either with hydrochloric acid or sodium hydroxide and then mixed in a ratio of 1:1 prior to digestion. This was in order to balance the pH of the sludge prior to digestion without the need for additional chemical dosing. Pretreatment periods of 1, 2 4, 6 and 8 days and 4, 22 and 37°C were tested. Following selection of optimum treatment conditions based on the pretreatment effect on sludge solids and COD concentrations, the study went on to considered the benefits to digestion at two sludge retention times.
Higher total and volatile solids reduction was achieved using NaOH for both the primary and secondary sludges, the degree of effectiveness being greater in the secondary sludge. The of number of pretreatment days did not significantly influence the degree of solids reduction but higher treatment temperature ie 37°C compared with 4°C or 22°C did bring some solids reduction benefit. An average of 53% VSS reduction was delivered by NaOH treatment of activated sludge at 37°C.
Total COD concentration was not affected by the chemical treatments in either sludges, however, soluble COD concentration was increased by both acid and base treatments indicating an improvement in cell hydrolysis. NaOH was again more effective than the HCl though the latter did increase soluble COD significantly in the activated sludge. The highest treatment temperature was again the most effective.
Volatile suspended solids removal increased by an average of 8% over the control digesters when activated sludge was pretreated with HCl and the primary sludge by NaOH. The 20 day SRT increased VSS reduction but there was no difference in the percentage improvement between different pretreatments. COD removal was similarly influenced by activated sludge treatment by acid and primary sludge treatment by the base, increasing by an average of 7% over the controls. Ammonia concentrations in the digesters were increased by chemical pretreatment, but there was no increase in the nitrogen concentration in the different treatment configurations indicating that there was no overall change in the nitrogen pathway in the digesters. Volatile fatty acid concentrations in the digesters were not influenced by chemical pretreatment and were at levels which indicated that good digester operating conditions were present.
Biogas production was not significantly influenced by the chemical pretreatments applied in this study, though the volume of methane produced was marginally lower in the control digesters. The acid and base used in this study were 5N NaOH and 5N HCl and the dosing rate used was 3% chemical by volume of each chemical which would reduce digester sludge capacity by 6%, as well an incurring substantial costs for the chemical sat an operational scale. Although chemical pretreatment did increase cell hydrolysis and solids reduction this was not reflected in increased biogas production.
Ozone
Ozone is a very strong oxidizing agent which, because of the complexity of sludge composition, reacts both directly and indirectly. It reacts directly and selectively with unsaturated bonds but it also decomposes and generates radicals that oxidize other organic matter. The potential for ozone treatment of sludge is significant since only 5% of the sludge is recalcitrant to ozonation (Déléris et al., 2000). During sludge pre-treatment, the aim of ozone is to cause the hydrolysis and partial oxidation of the organic matter. A complete oxidation is avoided. Ozone has an advantage over other chemical treatments because no chemicals are needed and no increase in salt concentration occurs (Goel et al., (2003).
Ozonation has be found to influence both biological and physicochemical characteristics of sludge. Floc size is reduced, particulate matter is solubilised, cell membranes destroyed as well as improved settleability, reduced odour and viscosity (Bougrier et al., (2007), Egemen et al., (2001), Weemaes et al., (2000))
Soluble and particulate compounds can be oxidized by ozone but the easily oxidizable soluble substances delay particle solubilisation (Cesbron et al., 2003). In addition, ozone will react with mineral compounds in sludge - about 20% of mineral solids can be solubilised this way (Salhi, 2003). Organic matter is also mineralised, an increasing ozone dose increasing COD mineralisation (Ahn et al., 2002). Consequently, ozone is used first for the solubilisation of compounds, then for both solubilisation of solids and mineralization of soluble organic compounds. However, the higher ozone doses required for mineralisation can have undesirable effects. Increasing ozone dose concentration causes a reduction in pH eg from 6.7 to 5.1 as ozone doses rose from 0.015 to 0.18 g O3/g TS indicating that ozone causes the oxidation of organic matter into more oxygenated molecules, such as carboxylic acids (Bougrier et al., 2007). Such pH drop may inhibit biological activity.
Biological wastewater treatment processes are less expensive than physico-chemical ones. Hence, to optimize the cost of a combined process, the biological step should be maximized and the chemical step minimized. When a process combines anaerobic digestion and ozonation, the objective is to convert solids into more easily biodegradable matter in order to facilitate treatment in the biological reactor thereby, increasing sludge biodegradability and biogas (Yeom et al., 2002).
Bourgier et al (2007) found that even low rates of ozone treatment increased biogas production volume and rates , though they were working with a sludge with inherently low biogas production potential. In contrast, Carballa et al., (2007) working with a sludge of high inherent degradability, a 70:30 mixture of primary and secondary sludge, rather than the WAS that was studied by Bougrier et al., (2007), found there to be no overall increase in solids destruction as a result of ozone treatment even though 60% of COD was solubilised by ozonation. Optimal ozone dosing for biogas production in the Bougrier et al., (2007) study was 0.15 g O3-1 g TS-1, which increased gas production by 2.4 times over the untreated control. Biogas production was directly related to sludge solubilisation, COD solids and nitrogen. At higher ozone dosing biogas volumes were reduced possibly indicating a pH effect or the creation of recalcitrant by-products.
Battimelli et al., (2003) looked at the effects of recycling increasing amounts of ozone treated sludge on sludge solids reduction and biogas formation. The ozone treatment was applied to the a portion of the digested sludge rather than raw sludge and that ozonated sludge was returned to the influent side of the digester. High ozone dosing (0.16gO3 g SS-1) gave good COD solubilisation in the digested sludge but reduced the filterability of the sludge due to the creation of small particle size. Sludge viscosity was reduced by high ozone dosing and settling characteristics were also improved. The ozone dosing rate which gave best COD solubilisation was selected to treat the sludge to be recycled, ie 0.16gO3 g SS-1, A recycling rate of 25% (by COD load) was found to be optimum, delivering increases in suspended solids and COD removal of 34 and 51% respectively compared with digestion without ozone treatment. The actual SS and COD removal compared with the influent sludge was 54 and 66% respectively with 25% recycling of ozonated digested sludge. Biogas production was not directly measured. Hydraulic retention time in the digester was reduced from 24 to 19 days by the same treatment configuration. The effects of recycling ozonated digested sludge on other sludge characteristics were not reported.
Cokgor et al., (2006) looked at ozone treatment of a penicillin effluent and achieved 70% greater COD removal plus substantially reduced inhibitory effect of the effluent on microbial degradation processes. This study suggests that ozone pretreatment may be particularly suited to WWTW with significant input from pharmaceutical industries or hospitals. However, Carballa et al., (2007) similarly looked at the effects of ozonation specifically on the removal of pharmaceutical and personal care products (PPCPs) and found there to be no benefit of ozone pretreatment to the elimination of these substances during AD. Calballa et al., (2007) were working with a 70:30 primary: secondary sludge mixture spiked with PPCPs which may have been more readily available for degradation that products which had passed through the wastewater treatment process.
Chelating agents such as EDTA and EGTA or sodium tripolyphosphate, STPP, have used for extraction of extracellular polymeric substances (EPS) and were shown to release increased amount of proteins, carbohydrates and humates present. Cation binding agents include substances such as formic acid, citric acid, tartaric acid, EDTA, sodium tripolyphosphate (STPP), Zeolite A, sodium fluoride, sodium thiosulphate and sodium silicate. Cation-binding agents disturb the sludge flocs structure by removing bridging ions (e.g. Ca2+, Mg2+, Fe2+ and Fe3+). The released organic matter as measured as COD can be available for anaerobic degradation and therefore the percent of inorganic matter in digested sludge would increase, leading to improved dewaterability (Wawrzynczyk et al 2007).
When enzymes are added to sludge they are often adsorbed on to the sludge matrix which can lead to inactivation. Treating the sludge with a cation binding agent prior to enzymatic treatment improves the organic matter in the sludge as a substrate for the enzymes through making components previously protected by the EPS structure more available to be degraded. (Wawrzynczyk, 2007)
Wawrzynczyk et al (2007) investigated the effects of chelating and ion-binding agents used alone and/or in combination with enzymes on solubilisation of a surplus biological sludge and a digested sludge both from municipal sources. Generally, the treatment of sludge with cation-binding agents led to high release of organic matter (measured as COD) and high changes in suspended matter (SS). The release of COD was positively dependent on concentration of cation-binding agent, i.e. the higher the concentration the more organic matter was released. In this study, the addition of an enzyme mix alone did not bring any treatment benefit to the sludge but the addition of the cation binding agent prior to enzyme addition increased the hydrolytic effect of the enzymes as indicated by COD solubilisation and increased the duration of effectiveness. Citric acid was the most effective agent and may be used operationally since it is completely biodegradable.
An enzyme is a molecule which catalyzes biological reactions. Nearly all known enzymes are proteins. There are six basic classes of enzymes; oxidoreductases, transferases, hydrolysases, lyases, isomerases and ligases. Enzymatic lysis is the cracking of the compounds of the cell wall by an enzyme catalysed reaction.
Hydrolytic enzymes are released from the microorganisms present in the sludge The rate of the hydrolysis reaction can be improved either by adding microorganisms producing hydrolytic enzymes to the sludge or by the direct addition of hydrolytic enzymes . Hydrolytic enzymes have several advantages over the use of microorganisms. They are cell free, small and soluble and are therefore able to reach the substrate easier. The enzymes can also function in the presence of microorganism predators and inhibitors of microbial metabolism and they function under a wide range of environmental conditions such as temperature and pH which might inhibit microbial activity. The enzymes also reduce the volume of the waste while microorganism addition contributes to biomass increasing the sludge volume (Wawrzynczyk et al., 2007, Beijer 2008).
Hydrolytic enzymes cause the solubilisation of extracellular polymeric substances in activated sludge. The increase in soluble COD is a direct measure of the degradation of suspended solids in sludge. Wawrzynczyk et al (2003) have shown that enzymatic treatment of sludge from Källby WWTP in Lund with four glycosidic enzymes, one lipase and one protease increased the release of soluble COD with increasing enzyme dose. The duration of a typical experiment was four hours and the temperature was kept at 45 °C and pH adjusted to maintain 7. Biogas production was increased by 60% by the addition of 60mg of each enzyme to 1g TS in the sludge. The same experiment indicated that enzymic action was improved at higher temperature and increased treatment time, though the majority of the enzyme action took place in the first few hours (Wawrzynczyk et al, 2003).
Davidsson et al. (2007) showed that the increase of methane production from enzyme treated sludge in general was higher in pilot scale continuous digestion than in batch digestion. The continuous digestion test also showed that a higher enzyme dose resulted in a significantly higher methane production while in the batch tests about the same methane production was achieved regardless of enzyme dose. Davidsson et al. (2007) suggested that this was because in the batch laboratory digestion tests the lower enzyme dose was optimal and with the higher dose the enzymes were active but there was no substrate available for them. In the continuous digestion test, fresh sludge is transferred to the digestion chamber every day so the enzymes are regularly supplied with new medium to act upon.
A full scale test over a six month period was conducted by Recktenwald et al. (2007). using two glycosidic enzymes of technical grade. The operation was continuous with a sludge feed of primary sludge mixed with biological. The dose of enzymes used was 2.5 kg of each enzyme solution per tonne feed TS to a digestion chamber with a daily feed load of 45m3 and retention time of 24 days. The control and treated digestion chambers were fed with the same amount and quality of sludge mixture from a buffer tank. The dosage point of enzymes was at a heat exchanger system which was run every fourth hour for 30 to 40 minutes. The digestion was carried out at 35°C but the heat exchange loop heated the sludge to 55°C. The enzymes selected had a temperature optimum between 45 - 60°C giving the enzymes an extra period of activation and mixing. The results from the full scale operation was improved gas production in the enzyme treated digestion chamber by 10 – 20% compared to the reference digestion chamber. No increase in VFAs was found (Recktenwald et al. 2007).
The enzymatic treatment of wastewater sludge has been shown to be significantly improved in the presence of cation binding agents (Wawrzynczyk et al. 2007b). The adsorption of the enzymes to the sludge matrix was reduced. A low dose of each enzyme (12 mg/g TS) was shown to be more effective in the presence of cation binding agents than a high dose of each enzyme (60 mg/g TS) alone. The most effective cation binding agent was proven to be citric acid for the tested substrates and there is also a potential for a practical application of this cation binding agent since citric acid is fully biodegradable (Beijer 2008).
Enzymatic lysis is also of interest in combination with mechanical disintegration because the intracellular liquid contains lysis enzymes. They can cause a further disintegration of the cells after mechanical treatment (Müller 2001, Dohanyos et al 1997).
It has been suggested that the application of enzymes for the treatment of primary sludge with a high content of lignocellulosic material is potentially more appropriate their use for waste activated sludges (Müller 2001). Applying enzymes to excess activated sludge can result in a fast degradation of the enzymes themselves before the enzymatic hydrolysis has started. Selection of appropriate enzymes for a particular sludge needs to be optimised to minimise any tendency for the enzymes to become bound in the sludge matrix . However, the cost of enzymes and cation binding agents is extremely high and the economic value of the increased gas yield and digester loading capacity gained by their use fall far short of paying for the necessary supply of enzymes. (Beijer 2008, Müller 2001)
Other biological pretreatment of sludge include the isolation of thermophilic aerobic bacteria, known to solubilise sludge solids, from an aerobic digestion reactor and the dosing of pretreated sludge with selected strains of clostridium to enhance methane and hydrogen production. Both these techniques are at early stages of development, and the latter is strictly not a pretreatment in as much at is relies on the application of one of the pretreatment methods to be applied first to be effective.
Solubilisation of organic sludge by thermophilic aerobic bacteria as a pretreatment for anaerobic digestion was investigate by Hasegawa et al (2000.)The bacterium type SPT2-1 could grow at pH ranging from 5.0 to 8.5 with optimal temperature at 60–70şC. In batch experiments, 25–30% of volatile suspended solids (VSS) in the pre-heated sludge were solubilised on inoculating with the isolated bacteria while little was solubilised without inoculation. The isolated bacteria appeared to secret the extracellular enzymes including proteases and amylases. In continuous flow experiments, VSS removal was around 40% under aerobic as well as micro-aerobic conditions. No accumulation of volatile fatty acids in the treated sludge was observed under aerobic conditions while significant amounts of them were accumulated under micro- aerobic conditions. Production of biogas on anaerobic digestion of the micro-aerobically pretreated sludge was increased by 1.5 when compared with the sludge without pretreatment (Hasegawa et al 2000).
It is possible to combine more than one pretreatment technique to further enhance the benefits to sludge degradation. Some studies have appeared to conduct a mix and match approach without any strong consideration being given to the synergies that might be exploited to optimise the overall outcome. Others have taken a more measured approach combining techniques which have complementary actions.
Kim et al (2003) considered the benefits of a range of pretreatment methods, individually and in combination and achieved between 2 and 12% greater COD solubilisation (as a measure of increase degradability) by following a thermal pretreatment with treatment with one of various alkalis, including NaOH, KOH, Mg(OH)a and Ca(OH)2. The optimal combination of thermal and alkali treatment was 121°C for 30 minutes followed by 7g/l NaOH which delivered a 85.4% increase in COD solubilisation. The same team found there to be no significant benefit in degradability from combining ultrasonic and thermal pretreatments over the individual treatment method.
One of the effects of many pretreatment processes is the degradation of cellular proteins and other nitrogenous organic material with the subsequent production of ammonical-N. Free ammonia is toxic to methanogenic bacteria and will inhibit methane production (Gallert and Winter 1997, Poggi-Varaldo et al. (1997), (Stroot et al. 2001), hence the very action of protein destruction achieved by many pretreatment processes can potentially prevent one of the primary objectives of conducting the pretreatment, namely to increase biogas production.
Cui and Jahng (2006) sought to control this potentially inhibitory effect of ammonium-N by removing solubilised protein following the application of three different pretreatments to WAS – ultrasound, thermal and alkali addition. Three methods were used to precipitate soluble proteins from the suspension of disintegrated sludge. There were salting out, using anhydrous magnesium chloride, calcium chloride or a mixture of the two with sodium citrate, known as Mix A, and glucono-d-lactone all of which are commonly used to precipitate Tofu from soya bean broth, isoelectric point precipitation to adjust the pH using either sulphuric acid or ammonium hydroxide, and heating. The combined effect of all three precipitation techniques was also tested.
The salting out technique achieved rates of soluble protein removal ranging from 49% w/w to 6% regardless of pH while thermal and isoprecipitation achieved 52% w/w soluble protein removal but only at pH3.3. Proteins recovered in this way have the potential to be reused eg for animal feed following further treatment (Cui and Jahng 2006). Comparison of the biogas yield from an untreated control with that of a sonicated sludge and a sonicated sludge which had been treated to precipitate the soluble protein found not only significant increase in biogas generation but that a greater proportion of that biogas was methane, ie 74.8% methane compared with 55.6% in the control. Whether the substantial increase in process complication that this two stage technique would require for full scale application would need to be assessed on a site by site basis. Also no testing of the dewaterabilty of the sludge post digestion was considered in this study.
A combination of thermal hydrolysis, ultrasonic and enzyme dosing were tested by Davidsson and Jansen (2006). Thermal hydrolysis is increasingly being employed in Swedish STWs for pathogen kill purposes in order to increase the acceptability of sludge use in agriculture to the food industry. The effects of this process on COD reduction and biogas yield in combination with ultrasound pretreatment and/or enzyme dosing of the digestion stage were studied at a bench scale using a biological sludge and a to a lesser extent, a mixed primary and biological sludge.
The ultrasound treatment (SonixTM 12 kW, 0.05 kWh/kg TS, Max. 50 kHz) was applied as an operational step at a full-scale treatment works and achieved a 1% increase in soluble COD while a 19% increase in soluble COD was achieved by thermal pretreatment employing 70°C for 1 hour. This suggests that the operational ultrasound treatment was breaking sludge flocs but was not sufficiently intensive to rupture cell walls. No benefit to COD solubility was obtained from combining thermal treatment with ultrasound or from heat treating the sludge after it had been previously treated using ultrasound. However, biogas yield from the ultrasound treated sludge was substantially higher than the untreated sludge despite the low COD solubilisation. In contrast, the biogas yield from the thermally hydrolysed sludge was not increased to the same degree as the COD was solubilised indicating that increased COD solubility is not automatically a good indicator of increased biogas yield.
Two enzyme mixtures were tested, by addition to the digester rather than as a pretreatment, to examine their effect on biogas yield. Mix A consists of four polysaccharide degrading enzymes and a lipase. Mix B contains protease, for complete hydrolysis of protein and glyco-proteins, and was separately added to avoid hydrolysis of enzymes in mix A during preparation and storage. The combination of enzyme addition to both ultrasound treated and thermally treated biological sludges delivered 45-50% more biogas than did the digestion of the biological sludge with no pretreatment. However, enzyme addition to the biological sludge receiving no pretreatment delivered only a small increase in the gas yield, around 10%.
The combination of chemical and thermal pretreatments has been invested by several researchers and found to be generally effective at both increasing COD solubilisation and biogas production. Penaud et al. (1999) studied the influence of sodium hydroxide addition during thermochemical pretreatment of industrial microbial biomass in terms of COD solubilisation and anaerobic biodegradability to enhance hydrolysis and complex polymers degradation. NaOH was added to sludge at concentrations varying between 0 and 26.1 g NaOH l-1. Based on the results of previous experiment, they treated half of each sample at 140°C for 30 minutes and the other half at ambient temperature.
At ambient temperature, COD solubilisation and TSS reduction increased as the NaOH dose increased. A 63% COD solubilisation and a 33% TSS reduction were achieved when a 4.6 g NaOH/L concentration was added. Increasing the alkali dose further resulted in a slower COD solubilisation and solids reduction. The maximum values were obtained for a 26.1 g NaOH/L with more than 75.4 COD solubilisation and more than 40% TS reduction. Without NaOH addition but treating the samples at 140 °C for 30 minutes resulted in COD solubilisation increasing from 37% to 80% and TS removal varied from 20 to 55%. For NaOH addition at 26.1 g NaOH l-1, 85.1% COD solubilisation and 80% TS reduction was obtained at 140°C. For both ambient and 140°C, the COD solubilisation variations were linked to pH patterns. The substantial alteration in pH brought about by high NaOH addition appeared to improve protein hydrolysis with a consequent increase of COD solubilisation.
The use of two or more pretreatments at an operational scale will potentially greatly increase the complexity of process control and the improvements in solids reduction and biogas production would have to be substantial to make the use of multiple pretreatments an economic and operationally practical option.
All pretreatment methods are designed to deliver the same outcome, that is, destruction of sludge flocs and cell walls and solubilisation of organic compounds in order to increase the degradability of the sludge. It can be seen from the preceding sections that the extent to which this is achieved varies greatly with the technique employed and the specific sludge to which it is applied. This appears to make is very difficult to determine what is the most appropriate technique to select for any given circumstance. The following section tries to make broad comparison of the principle methods though consideration of their net energy use, affect of key sludge characteristics. The level of commercial deployment is considered in Section 10. Most of the information is taken from Muller 2001 and STOWA
The performance of the various methods can be compared by the specific energy, which is defined as the amount of mechanical energy that stresses a certain amount of sludge. Of the mechanical methods, the lysate centrifuge shows the lowest energy consumption, the ultrasonic homogeniser the highest. However, the high pressure homogeniser, stirred ball mill and ozone treatment achieve medium degrees of disintegration with a relatively low energy input. Thermal treatment uses more energy, but thermal energy is often cheaper than the electrical energy that is necessary for the other methods and the use of heat exchangers allow recovery and reuse of a significant proportion.
Techniques which only disrupt sludge flocs have little influence on the digestion process. High energy techniques, whether thermal or mechanical, which bring about good cellular degradation rate can significantly increase in biogas production Thermal disintegration within the range of 135 - 180°C was found to be optimal with regard to gas production. Disintegration and gas yield can be increased significantly by ozonation though the active anaerobic biomass has first to adapt to the changed substrate before benefits are obtained. For the use of enzymes, an increase in degradation of about 10% has been reported, but the results of various investigations have been inconsistent.
The operational uptake of pretreatments techniques has been relatively slow despite over 10 years of research much of which has shown there to be potentially large benefits to gas production and solids reduction from all the main technologies. High capital costs and long pay back periods, high consumption of energy and/or chemicals, significant operating problems (such as odour, corrosion, and down time due to high maintenance demands as well as problems scaling up technologies have slowed or stopped commercial development.
Operational adoption has tended to be focused on problem works, where more conventional methods of treatment have consistently failed to deliver satisfactory operational conditions, or effluent or sludge quality. Full-scale testing of most techniques has been limited and operational installations are largely limited to ultrasonic techniques or mechanical disintegration by lysate-thickening centrifuge (Zábranská et al 2006). In the UK, thermal hydrolysis in the form of the Cambi process has been making inroads to the water industry in recent years as can be seen from the list of UK installations.
Pressures on sludge outlets, the increase in sludge make caused by higher discharge consents and energy costs, both financial and environmental are all stimulating a renewed interest in sludge pretreatment options.
Cambi [http://www.cambi.no]
Sludge is dewatered to 16-17% dry solids (DS) and stored in a silo from where it is fed into a pulper to be mixed and heated by recycled steam from the reactor and the flash tank. Thermal hydrolysis takes place under pressure in the reactor at 165şC for 20-30 minutes. The pasteurised sludge is then passed rapidly into the flash tank, the resulting pressure drop causing cell rupturing. The sludge is then cooled to the required digestion temperature partly by adding dilution water and partly in the heat exchangers.
The steam for the thermal hydrolysis is mainly produced in a cogeneration waste heat boiler using exhaust gas and cooling water from the gas engine. Alternatively, biogas or other fuel sources can be used.
The thermal hydrolysis process (THP) allows much higher solids loading into digesters – around 10.5% and volatile solids concentrations >6kgVSm-3 day-1, which an average increase in digester capacity of 300% and typically delivering VS destruction rates of 60%. This increases effectively digester capacity by threefold which can avoid the need to build new digester capacity. The higher WS destruction brings substantial increase in biogas production and dewatering characteristics are improved.
Baker Process [http://www.lysate-centrifuges.com]
A special beater or lysate ring, integrated into the centrifugal thickener, dissipates the kinetic energy provided by the centrifuge. The sludge is not milled but cell structures are destroyed by deceleration forces imparted by the beater design, impact forces as a result of collision of particles in the centrifuge space, and additional shear effects in the upper structure due to the centrifuge rotation rate in the range of 1500 to 3000 rpm. The process aims to destroy 10 - 20% of the cells which will give up to 25% increase in biogas yield. The technique does not affect the dewatering characteristics of the sludge.
Cell destruction takes place in the centrifuge effluent following thickening, thus the centrate load is not any higher than in normal centrifugation. The installation of the Lysate device takes place within the centrifuge housing and can be retrofitted to existing machines. The increase in energy consumption for the additional shearing forces is estimated at 20%, but overall the disintegration ratio related to the specific energy consumption (kWh kg-1 SS) is relatively low compared to other disintegration methods.
Bugbuster [http://www.ecosolids.com/bugbuster.htm]
Bugbuster is a new technology which underwent six months of trials at Yorkshire Water’s Whittington WWTW in 2007.The process pumps CO2 derived from biogas into a sludge filled reactor at 4 bar pressure where it permeated through the floc and dissolves over a period of minutes. When the pressure is released the CO2 rapidly expands rupturing floc particles and cell walls. It is claimed that the system will result in 40% more biogas and that the cost of installation will be recovered within two years.
Bioleader process [http://www.kurita.co.jp/english]
Bioleader is a patented, full scale ozonation process developed in Japan. The process works by withdrawing sludge from the aeration tank or secondary clarifier and passing it through an ozone contactor before recirculating it to the aeration tank. The treatment of three times the recirculated sludge volume results in zero excess sludge.
The process is designed to deliver high absorption rates of ozone by the sludge under high-rate loading, energy efficient ozone production and the suppression of foaming to prevent clogging of the ozone distribution. About 0,05 to 0,15 kg O3/kg DS is required dependent on the type of chemicals used elsewhere in the treatment process. The ozone is produced from air, requiring about 15 to 20 kWh/kg O3.
The operating costs are mainly associated with electricity consumption in ozone production and additional aeration of aeration tanks. The energy consumption is in the order of 0,6 to 1,8 kWh/ kg dry solids (DS). Capital costs for a 100,000pe treatment works would be in the region of 90-470/ t DS (Stowa 2006). A Bioleader installation can have implications for effluent quality and particular attention is needed with regard to N and P removal.
OpenCEL
OpenCEL utilises a rapidly pulsing, high-voltage electric field – 20 to 30 kV - applied over milliseconds to disrupt and break up cellular membranes and cell walls, complex organic solids, and macromolecules. It is an adaptation of pulsed electric field (PEF) technology, which is used in molecular biology for electroporation, the reversible opening of pores in cellular membranes to perform plasmid and DNA insertions and for medical therapies. PEF also is widely used by food biologists as an alternative to traditional pasteurization technologies (Rittman 2008). The technique has been applied at full scale to a mixture of thickened primary sludge and WAS. Treating 48-63% of the sludge volume delivered 40% more biogas and 25-30% less sludge for disposal over a six month period (Rittman 2008).
The pulse technology also causes an increase in sludge temperature such that sludge entering the digester is 11°C on average warmer than when there is no pulse power applied, providing savings in digester heating. This coupled with the additional biogas results in 18 times more energy benefit than the energy used by OpenCEL. Installed at a 76,000m3 day-1 WWTW, it is expected that the installation would pay for itself within three years due to increased biogas and reduced sludge management costs (Rittman 2008).
A number of broad conclusions can be drawn from the above overview of the development stages of pretreatment technologies for sludge.
Pretreatment efficiency appears to increase with concentration of sludge solids hence it may be most efficient to apply these technologies to thickened sludge.
Benefits of the improved hydrolysis brought about by pretreatment techniques are greatest when applied to biological sludges. Hence, in economic and operational terms, it may be worth considering only applying pretreatment to activated sludge streams rather than mixed sludge.
Pretreatment appears to more effectively disintegrate young WAS than old WAS which should be taken into account in the design of an installation.
Pretreatments can be applied where any AD process is in place.
With the possible exception of thermal pretreatment, pathogen kill is generally not improved by pretreatment techniques.
It is not appropriate to use solely use surrogates such as COD solubilisation or VSS removal as indicators for increased biogas production since many studies have shown that increased COD solubilisation seen after pre-treatment does not always bring about an increase in biogas yield. On the other hand, pre-treatment can lead to higher methane production although the COD solubilisation is low or not markedly increased.
The pattern of gas production is changed by pretreatment in that the rate of gas production increases more rapidly in pretreated sludges, so that in the early period of monitoring it appears that the increase in gas production is very marked. However, if monitoring continues for a longer period the gas yield from untreated sludge often reaches similar rates, but just more slowly. Consequently, some of the claims made by researchers for particular techniques are potentially exaggerated because the period of monitoring looked only at the early stages of digestion over a few days rather than over a full operational sludge retention period.
Many of the pretreatments tested have been targeted at achieving a particular effect within the body of the sludge but have not thought about the potential additional effects, which are ultimately counterproductive. For example, the application of excessive amounts of heat leads to the denaturing of proteins in the sludge which actually makes them more resistant to digestion processes.
The benefits of pretreatment appear to be very sludge specific and even where sludges should be similar having been derived from similar treatment processes, their response to a specific pretreatment can differ widely. Hence, it is suggested that if pretreatment is being for a particular treatment work, it would be advisable to bench and/or pilot test candidate technologies in order to identify that which will deliver the best overall benefit for sludge management.
The vast majority of studies are conducted on a small scale using only 1 to 3l of sludge per test chamber. This permits very close control of conditions and thorough penetration of the applied pretreatments. Consequently, conditions are optimum for achieving maximum effectiveness of the applied treatment. The large beneficial impacts that are reported in these studies are generally not repeated when the technique is tested at an operational scale. Additionally, many of the techniques have not actually been applied or tested operationally.
Provided the net energy balance is still positive then the adoption of sludge pretreatment is a better option to adopt than sludge incineration which is not only publically unacceptable but can only be applied at very large works.
Many of the techniques investigated are extremely complex and would be difficult to apply operationally. In many cases, the benefits seem small for increased process complication that would result from their adoption. However, with continuing legislative pressures driving up sludge production while at the same time making disposal of that sludge more difficult, wider operational development of sludge pretreatment would be highly beneficial to the water utilities.
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STOWA [http://www.stowa-selectedtechnologies.nl]
The Dutch Foundation for Applied Water Research (STOWA) commissioned a project completed in October 2002 into commonly used as well as newly developed techniques in the field of wastewater treatment. The search focused on techniques, which are used abroad and not used in the Netherlands, and which are assessed for its feasibility in the Netherlands. Altogether 52 techniques were chosen for further investigation. For each technique a fact sheet has been produced, which contains information on the background of the technique, a description of the principle, design guidelines and technical data, performance, operational stability and maintenance, capital and operating cost, reference installations, suppliers and patents and literature references.
Cambi [http://www.cambi.no]
Baker Process [http://www.lysate-centrifuges.com]
Bugbuster [http://www.ecosolids.com/bugbuster.htm]
Bioleader process [http://www.kurita.co.jp/english]
