Anaerobic membrane bioreactors may provide a sustainable technological solution for digestion of waste activated sludge due to their capacity to achieve substantial volatile solids (VS) destruction and positive energy balances with reduced digester volumes. However, membrane integrated anaerobic systems may have limitations that are imposed by membrane fouling and a decrease in biomass activity due to possible exposure of biomass to high shear conditions. This study characterised bioprocess and membrane performance under varying conditions, identified foulant type and origin and mechanism of fouling, and developed fouling control strategies by using low cross flow velocity and pressure anaerobic membrane systems. The study employed a pilot scale anaerobic digester integrated with negative and neutral tubular membranes; pilot and bench scale control digesters supported with bench scale filtration unit parametric studies. The membranes were polyvinylidene difluoride based with an average pore size of 0.02 micron and were operated at a constant cross flow velocity of 1 ms-1 and constant trans-membrane pressure of 30 kPa. Four operating conditions consisting of different combinations of HRT and SRT were evaluated. By integrating membranes into the digesters it was possible to simultaneously enhance digestion and increase throughput of the digesters without affecting its performance. The anaerobic membrane digester showed 48-49% volatile solids destruction at 30 days SRT under conventional and higher loadings of 1.2±0.4 and 2.1±0.6 kg COD m-3day-1. This was a 100% increase in performance compared to a control digester subjected to higher loading. This result was supported by the associated specific methane generation. The control digesters operated at a relatively higher SRT showed comparable VS destruction and gas generation to the anaerobic membrane running at a similar SRT. However the extra gas generated didn’t compensate heat required to maintain larger volume of the digester. In case of anaerobic membrane digesters due to the high rate feeding, increase biogas production and co-thickening, the energy balance increased by 144 and 200% under conventional and higher loading conditions respectively. Characterization of membrane performance showed that the average sustainable flux was 23.2±0.4 and 14.8±0.4 LMH during HRT-SRTs of 15-30 and 7-15 days respectively. The critical fluxes were in the range of 30-40, 16-17 and 20-22 LM-2H-1 during HRT-SRTs of 15-30, 7-30 and 7-15 days respectively. The decline in membrane performance at a higher loading was associated with the formation of cake layers on the membrane surface that led to reversible fouling. The additional decline in performance at extended SRT was attributed to irreversible fouling. The colloidal fraction of the sludge showed an overall higher fouling propensity during the long term pilot studies and short term filtration tests. The suspended solids fraction of the sludge showed a positive impact at concentration below 15 g/L but resulted in a decrease of membrane performance at higher concentrations. Further studies of foulant origin through a series of microscopic, membrane cleaning and sludge characterization studies showed that the colloidal proteins, soluble carbohydrates and inorganic materials such as iron, calcium and sulfur and their interaction to have a significant impact on membrane fouling. To control anaerobic membrane fouling by the digested sludge, integration of membrane relaxation techniques in the filtration cycle were found effective. By incorporating a unique relaxation technique to tubular membranes, it was possible to increase the sustainable flux to 29.2±1.8 and 34.5±2.5 LM-2H-1 for neutral and negative membranes during 15-30 HRT-SRT process condition. Addition of cationic polymers and sequential mechanical-citric acid membrane cleaning, that targeted both reversible and irreversible fouling was also found effective.

Anaerobic membrane bioreactors may provide a sustainable technological solution for digestion of waste activated sludge due to their capacity to achieve substantial volatile solids (VS) destruction and positive energy balances with reduced digester volumes. However, membrane integrated anaerobic systems may have limitations that are imposed by membrane fouling and a decrease in biomass activity due to possible exposure of biomass to high shear conditions. This study characterised bioprocess and membrane performance under varying conditions, identified foulant type and origin and mechanism of fouling, and developed fouling control strategies by using low cross flow velocity and pressure anaerobic membrane systems. The study employed a pilot scale anaerobic digester integrated with negative and neutral tubular membranes; pilot and bench scale control digesters supported with bench scale filtration unit parametric studies. The membranes were polyvinylidene difluoride based with an average pore size of 0.02 micron and were operated at a constant cross flow velocity of 1 ms-1 and constant trans-membrane pressure of 30 kPa. Four operating conditions consisting of different combinations of HRT and SRT were evaluated. By integrating membranes into the digesters it was possible to simultaneously enhance digestion and increase throughput of the digesters without affecting its performance. The anaerobic membrane digester showed 48-49% volatile solids destruction at 30 days SRT under conventional and higher loadings of 1.2±0.4 and 2.1±0.6 kg COD m-3day-1. This was a 100% increase in performance compared to a control digester subjected to higher loading. This result was supported by the associated specific methane generation. The control digesters operated at a relatively higher SRT showed comparable VS destruction and gas generation to the anaerobic membrane running at a similar SRT. However the extra gas generated didn’t compensate heat required to maintain larger volume of the digester. In case of anaerobic membrane digesters due to the high rate feeding, increase biogas production and co-thickening, the energy balance increased by 144 and 200% under conventional and higher loading conditions respectively. Characterization of membrane performance showed that the average sustainable flux was 23.2±0.4 and 14.8±0.4 LMH during HRT-SRTs of 15-30 and 7-15 days respectively. The critical fluxes were in the range of 30-40, 16-17 and 20-22 LM-2H-1 during HRT-SRTs of 15-30, 7-30 and 7-15 days respectively. The decline in membrane performance at a higher loading was associated with the formation of cake layers on the membrane surface that led to reversible fouling. The additional decline in performance at extended SRT was attributed to irreversible fouling. The colloidal fraction of the sludge showed an overall higher fouling propensity during the long term pilot studies and short term filtration tests. The suspended solids fraction of the sludge showed a positive impact at concentration below 15 g/L but resulted in a decrease of membrane performance at higher concentrations. Further studies of foulant origin through a series of microscopic, membrane cleaning and sludge characterization studies showed that the colloidal proteins, soluble carbohydrates and inorganic materials such as iron, calcium and sulfur and their interaction to have a significant impact on membrane fouling. To control anaerobic membrane fouling by the digested sludge, integration of membrane relaxation techniques in the filtration cycle were found effective. By incorporating a unique relaxation technique to tubular membranes, it was possible to increase the sustainable flux to 29.2±1.8 and 34.5±2.5 LM-2H-1 for neutral and negative membranes during 15-30 HRT-SRT process condition. Addition of cationic polymers and sequential mechanical-citric acid membrane cleaning, that targeted both reversible and irreversible fouling was also found effective.

In recent years there has been pressure exerted on the feed industry in Southern Africa to produce enough animal feed to meet the region’s nutritional requirements. The increase in the cost of animal feed eventually affects the affordability and availability of high quality food to low income communities. However, the overall national production of protein feed can easily be surpassed by the amount of protein that could be extracted from sludge. For example, the amount of protein wasted through sludge in one province alone (Gauteng, South Africa) amounts to 106,763 metric tonnes/yr, and slightly lower than the national protein requirement of approximately 145,000 tonnes/yr. Waste Activated Sludge (WAS) from wastewater treatment plants treating domestic wastewater is shown to contain protein in a ratio of 2:1 against fishmeal. However, some of this protein content could be lost during processing. In this study, the protein content in sludge and fishmeal was evaluated in laboratory analyses conducted as a preliminary step towards designing a protein supplement substitute. A pilot test was conducted with 5 batches (10 chicken per batch), with fishmeal to sludge substitutions of 0%, 25%, 50%, 75%, and 100%. Metal content in the sludge was lowered by a rudimentary leaching process and its impact on the protein content was also evaluated. The initial mass gain rate, mortality rate, initial and operational costs analyses showed that protein from Waste Activated Sludge (WAS) could successfully replace the commercial feed supplements with a significant cost saving without adversely affecting the health of the animals. Copyright / Dissertation (MSc)--University of Pretoria, 2010. / Chemical Engineering / unrestricted

<p>Biogas, a mixture of methane and carbon dioxide, is produced in the anaerobic digestion of sewage sludge. After anaerobic digestion, the digested sludge is often allowed to degas for one or two days. This gas is seldom utilised, but if the degassing could be accelerated, utilisation would be easier. Ultrasound can be used as a pretreatment method for waste activated sludge. It has a disintegrating effect on the sludge and causes lysis of bacteria in the sludge. It also speeds up the hydrolysis; the limiting step of anaerobic digestion of waste activated sludge. Ultrasound can be used to degas waterbased liquids. Ultrasonic degassing of sewage sludge has not been examined previously. The present study aims to investigate the effect of ultrasound on waste activated sludge as well as the potential of ultrasound to speed up the degassing of digested sludge. A semi-continuous, lab-scale digestion experiment was performed with four reactors: two receiving untreated sludge and two receiving treated sludge. The effect of the sonicator was 420 W and the treatment time was 6 min, which corresponds to an energy input of 8.4 kWh/m³. Total solids (TS) of the waste activated sludge was ~3.5 %. The ultrasonic treatment caused an increase in gas production of 13 %. There was no difference in methane content. The concentration of filterable chemical oxygen demand (fCOD) increased 375 %, or from 2.8 % to 11 % of total COD. In terms of energy loss/gain the increase in gas production resulted in a loss of 2.7 kWh/m³, i.e. more energy is needed to treat the sludge than the potential energy of the increased gas production. However, if the sludge is thickened to a TS >5 %, a net energy gain should be reached. The effect of ultrasound on the degassing of digested sludge was examined in three barrels. The degassing was measured with and without circulation as well as with ultrasonic treatment. The digested sludge had a gas emission rate of 115 L/(m³ day). No direct burst of gas occurred due to ultrasonic treatment. Over two days more gas was emitted from the barrel equipped with ultrasound, probably due to an induced post-digestion. Thus, ultrasonic pretreatment of waste activated sludge increases the biogas yield. It is inconclusive, whether ultrasonic treatment of digested sludge effects the degassing or not.</p>

Anaerobic digestion of excess biological wastewater treatment sludge (WAS) from pulp mills has the potential to reduce disposal costs and to generate energy through biogas production. The organic matter in WAS is highly structured, which normally hinders biogas production. This study investigated three methods of pretreating WAS from two different pulp mills before anaerobic digestion to improve biogas yield and production rate. The three pretreatment methods tested were: i) thermal pretreatment at 170oC, ii) caustic pretreatment at 140oC and pH 12, and iii) sonication at 20 kHz and 1 W/mL. Thermal pretreatment proved to be the most effective, increasing biogas yield by 280% and 50% and increasing production rates 300-fold and 10-fold for the two samples, respectively. Caustic pretreatment showed similar results, but resulted in the formation of soluble non-biodegradable compounds. Sonication was the least effective pretreatment and did not substantially increase biogas yield, but increased biogas production rate.

Anaerobic digestion of excess biological wastewater treatment sludge (WAS) from pulp mills has the potential to reduce disposal costs and to generate energy through biogas production. The organic matter in WAS is highly structured, which normally hinders biogas production. This study investigated three methods of pretreating WAS from two different pulp mills before anaerobic digestion to improve biogas yield and production rate. The three pretreatment methods tested were: i) thermal pretreatment at 170oC, ii) caustic pretreatment at 140oC and pH 12, and iii) sonication at 20 kHz and 1 W/mL. Thermal pretreatment proved to be the most effective, increasing biogas yield by 280% and 50% and increasing production rates 300-fold and 10-fold for the two samples, respectively. Caustic pretreatment showed similar results, but resulted in the formation of soluble non-biodegradable compounds. Sonication was the least effective pretreatment and did not substantially increase biogas yield, but increased biogas production rate.

Biogas, a mixture of methane and carbon dioxide, is produced in the anaerobic digestion of sewage sludge. After anaerobic digestion, the digested sludge is often allowed to degas for one or two days. This gas is seldom utilised, but if the degassing could be accelerated, utilisation would be easier. Ultrasound can be used as a pretreatment method for waste activated sludge. It has a disintegrating effect on the sludge and causes lysis of bacteria in the sludge. It also speeds up the hydrolysis; the limiting step of anaerobic digestion of waste activated sludge. Ultrasound can be used to degas waterbased liquids. Ultrasonic degassing of sewage sludge has not been examined previously. The present study aims to investigate the effect of ultrasound on waste activated sludge as well as the potential of ultrasound to speed up the degassing of digested sludge. A semi-continuous, lab-scale digestion experiment was performed with four reactors: two receiving untreated sludge and two receiving treated sludge. The effect of the sonicator was 420 W and the treatment time was 6 min, which corresponds to an energy input of 8.4 kWh/m3. Total solids (TS) of the waste activated sludge was ~3.5 %. The ultrasonic treatment caused an increase in gas production of 13 %. There was no difference in methane content. The concentration of filterable chemical oxygen demand (fCOD) increased 375 %, or from 2.8 % to 11 % of total COD. In terms of energy loss/gain the increase in gas production resulted in a loss of 2.7 kWh/m3, i.e. more energy is needed to treat the sludge than the potential energy of the increased gas production. However, if the sludge is thickened to a TS &gt;5 %, a net energy gain should be reached. The effect of ultrasound on the degassing of digested sludge was examined in three barrels. The degassing was measured with and without circulation as well as with ultrasonic treatment. The digested sludge had a gas emission rate of 115 L/(m3 day). No direct burst of gas occurred due to ultrasonic treatment. Over two days more gas was emitted from the barrel equipped with ultrasound, probably due to an induced post-digestion. Thus, ultrasonic pretreatment of waste activated sludge increases the biogas yield. It is inconclusive, whether ultrasonic treatment of digested sludge effects the degassing or not.

Waste activated sludge (WAS) pretreatment technologies are typically evaluated in terms of the associated improvement in biogas and sludge production during digestion and post-digestion dewaterability. However, WAS properties, and hence the impact of pretreatment on WAS properties, are dependent upon the raw wastewater composition and configuration of the wastewater treatment plant (WWTP). A generally accepted means of characterizing and comparing all pretreatment processes does not exist. The motivation for this project was to evaluate the impact of pretreatment on WAS properties in terms of changes in COD fractionation. The first objective of this study was to fractionate the COD of the WAS before and after pretreatment to show how pretreatment may increase the rate and extent of aerobic digestion. The second objective was to develop a COD-based stoichiometric pretreatment model that may be integrated into WWTP simulations. A bench-scale biological reactor (BR) with a solids retention time (SRT) of 5 days was started up with WAS from the Waterloo WWTP. The BR was fed daily with a completely biodegradable synthetic substrate so that the BR WAS contained only biomass and decay products after 3 SRTs of operation. In the first phase of the study, an aerobic digester (AD) with a SRT of 10 d was fed daily with BR WAS. The BR-AD system was operated at steady state for one month. A range of physical and biochemical properties were regularly measured in each process stream. Offline respirometric tests were regularly conducted to determine the aerobic degradability and fractionate the COD of the BR and AD WAS. The oxygen uptake rate (OUR) associated with the daily addition of BR WAS to the AD was determined as an additional measurement of the aerobic degradability of the BR WAS. In the second phase of the study, the BR WAS was pretreated prior to being fed daily to the AD. High pressure thermal hydrolysis (HPTH) pretreatment was selected for this project since it is one of the most popular and promising pretreatment techniques. A sealed volume of BR WAS was heated to 150°C at 3 bars for 30 minutes. The same physical, biochemical and biological tests used to characterize the process streams in Phase 1 were employed to characterize those in Phase 2. The Phase 2 system was operated for two months at steady-state. The results of several independent tests showed that the COD of the BR WAS was comprised of storage products (XSTO) in addition to active heterotrophs (Zbh) and decay products (Ze). However, it was shown that the AD WAS only contained Zbh and Ze as XSTO was depleted in the AD. HPTH pretreatment did not reduce the TCOD concentration of the WAS however it did solubilize 56 ± 7% of COD, 49% ± 11% of organic nitrogen, 56 ± 10% of VSS and did not solubilize ISS. Furthermore, pretreatment did not generate soluble non-biodegradable COD. These findings were consistent with prior research on HPTH WAS pretreatment. Pretreatment increased the rate at which the BR WAS was aerobically degraded. The offline respirometric tests showed that the pretreated BR WAS contained a substantial amount of readily biodegradable COD (Sbsc). However, pretreatment did not increase the extent of biodegradation. The results of several independent tests showed that the non-biodegradable COD component of the BR WAS, i.e. Ze, was not converted to biodegradable COD by pretreatment. A COD-based stoichiometric pretreatment model was developed for the dose of HPTH pretreatment employed in this study. When this model was integrated into BioWin®, it was able to accurately simulate both the steady state performance of the overall system employed in this study as well as dynamic respirometry results. The experimental results showed that the TCOD of the BR WAS consisted of 51% Zbh, 12% Ze and 37% XSTO and the pretreated BR WAS consisted of 12% Ze and a negligible amount of Zbh. The pretreatment model verified these fractions and predicted that the pretreated BR WAS also contained 54% Sbsc and 32% slowly biodegradable COD (Xsp). The approach described in this study may be followed to determine the impacts of pretreatment on Zbh, Ze and XSTO when other doses of HPTH pretreatment and other pretreatment techniques are employed.

Waste activated sludge (WAS) pretreatment technologies are typically evaluated in terms of the associated improvement in biogas and sludge production during digestion and post-digestion dewaterability. However, WAS properties, and hence the impact of pretreatment on WAS properties, are dependent upon the raw wastewater composition and configuration of the wastewater treatment plant (WWTP). A generally accepted means of characterizing and comparing all pretreatment processes does not exist. The motivation for this project was to evaluate the impact of pretreatment on WAS properties in terms of changes in COD fractionation. The first objective of this study was to fractionate the COD of the WAS before and after pretreatment to show how pretreatment may increase the rate and extent of aerobic digestion. The second objective was to develop a COD-based stoichiometric pretreatment model that may be integrated into WWTP simulations. A bench-scale biological reactor (BR) with a solids retention time (SRT) of 5 days was started up with WAS from the Waterloo WWTP. The BR was fed daily with a completely biodegradable synthetic substrate so that the BR WAS contained only biomass and decay products after 3 SRTs of operation. In the first phase of the study, an aerobic digester (AD) with a SRT of 10 d was fed daily with BR WAS. The BR-AD system was operated at steady state for one month. A range of physical and biochemical properties were regularly measured in each process stream. Offline respirometric tests were regularly conducted to determine the aerobic degradability and fractionate the COD of the BR and AD WAS. The oxygen uptake rate (OUR) associated with the daily addition of BR WAS to the AD was determined as an additional measurement of the aerobic degradability of the BR WAS. In the second phase of the study, the BR WAS was pretreated prior to being fed daily to the AD. High pressure thermal hydrolysis (HPTH) pretreatment was selected for this project since it is one of the most popular and promising pretreatment techniques. A sealed volume of BR WAS was heated to 150°C at 3 bars for 30 minutes. The same physical, biochemical and biological tests used to characterize the process streams in Phase 1 were employed to characterize those in Phase 2. The Phase 2 system was operated for two months at steady-state. The results of several independent tests showed that the COD of the BR WAS was comprised of storage products (XSTO) in addition to active heterotrophs (Zbh) and decay products (Ze). However, it was shown that the AD WAS only contained Zbh and Ze as XSTO was depleted in the AD. HPTH pretreatment did not reduce the TCOD concentration of the WAS however it did solubilize 56 ± 7% of COD, 49% ± 11% of organic nitrogen, 56 ± 10% of VSS and did not solubilize ISS. Furthermore, pretreatment did not generate soluble non-biodegradable COD. These findings were consistent with prior research on HPTH WAS pretreatment. Pretreatment increased the rate at which the BR WAS was aerobically degraded. The offline respirometric tests showed that the pretreated BR WAS contained a substantial amount of readily biodegradable COD (Sbsc). However, pretreatment did not increase the extent of biodegradation. The results of several independent tests showed that the non-biodegradable COD component of the BR WAS, i.e. Ze, was not converted to biodegradable COD by pretreatment. A COD-based stoichiometric pretreatment model was developed for the dose of HPTH pretreatment employed in this study. When this model was integrated into BioWin®, it was able to accurately simulate both the steady state performance of the overall system employed in this study as well as dynamic respirometry results. The experimental results showed that the TCOD of the BR WAS consisted of 51% Zbh, 12% Ze and 37% XSTO and the pretreated BR WAS consisted of 12% Ze and a negligible amount of Zbh. The pretreatment model verified these fractions and predicted that the pretreated BR WAS also contained 54% Sbsc and 32% slowly biodegradable COD (Xsp). The approach described in this study may be followed to determine the impacts of pretreatment on Zbh, Ze and XSTO when other doses of HPTH pretreatment and other pretreatment techniques are employed.

A big challenge within pulp and paper mills is the large quantities of waste activated sludge (WAS) that is produced during the wastewater treatment. The WAS is made up of biological cells and extra polymeric substances (EPS) and can bind a large amount of water causing difficulties to dewater the WAS. This study aimed to determine how to improve the dewatering properties of the WAS by using hydrolysis. Hydrolysis will cause the cells to disrupt and the bound water in the cells and the water trapped by the EPS can be released. Specifically, this study investigated what impact hydrolysis with heat, alkalis, and acids had on the WAS dewatering properties. In addition to the impact on the dewatering properties, the release of organic material and nutrients from the cells has also been important for biomethane production. In this study, it was specifically NH4-N, PO43- and COD that have been studied. WAS from paper mills have in general poor methane potential so it was of interest to see how the WAS was affected by hydrolysis and how hydrolysis could improve the methane production. To test the hypothesis of whether hydrolysis could affect the WAS and improve the dewatering properties, several experiments were performed. The experiments included thermal hydrolysis at temperatures of 70-90 °C, acidic hydrolysis with acids such as spent acid and acid water, and alkalis such as green liquor sludge and EOP. All acids and alkalis used in the study were chemicals that exist at the paper mills included in this study. To test the dewatering properties, methods such as TS analysis on the accept, CST-analysis, and a belt press were used. Analyses were also performed on the reject to measure the suspended solids and the nutrients NH4 – N, PO43– and COD in the WAS. This study did also determine what effect hydrolysed WAS had on the biomethane potential. In this study were the paper mills BillerudKorsnäs in Skärblacka and SCA in Östrand included. Hence was sludge from the two mills of interest to analyse. This study has shown better dewatering properties with an increase in the total solids (in the accept) after the thermal hydrolysis, the acidic hydrolysis with spent acid, and the alkali hydrolysis with green liquor sludge. Specifically did the acidic hydrolysis with spent acid improve the dewatering properties in terms of an increase in TS in %. The biggest increase in TS in % could be seen after using 10% spent acid ratio. The TS for the WAS from SCA Östrand increased in this experiment by 107 %. The thermal hydrolysis also showed promising results both in terms of dewatering properties and in the release of organic material. The biochemical methane potential test results showed a better and more rapid stabilized production of biomethane after hydrolysis of WAS compared to untreated WAS. The thermal hydrolysis both increased the rate of production and the total amount of methane produced. The thermally hydrolysed WAS from SCA Östrand improved the methane production from 77 Nml methane/g VS to 95 Nml methane/ g VS. The WAS from BillerudKorsnäs improved the methane production from 40 Nml methane/ g VS to 55 Nml methane/ g VS. These results, both from the methane potential tests and the results of the increased dewatering properties, show that the concept with hydrolysing should be evaluated further for improving the dewatering of the WAS.