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This study aims to characterize anaerobically digested sludge (ADS) and correlate the sludge characteristics in terms of soluble organic compounds with polymer demand (PD) during sludge conditioning. The PD required to achieve maximum dewatering of the ADS studied is in the range of 8–10 kg polymer/dry ton. The commonly used capillary suction time parameter to evaluate the solid–liquid separation ability was not a reliable indicator for assessing dewatering. Instead, in this study, a modified centrifugal technique proposed by Higgins (Higgins MCT) was used to assess the maximum achievable dry solids content of the biosolids cake. The Higgins MCT is readily obtained using a bench-scale centrifuge equipped with a modified centrifuge bucket. Using the Higgins MCT, the maximum dry solids contents obtained from conditioned ADS was 30 wt%. These values were comparable to the dry solids content obtained from the same sludge at full-scale level. Our results suggest Higgins MCT is suitable for assessing the final dry solids content and simulating the dewatering process. [ABSTRACT FROM AUTHOR]

In this study, high-solids anaerobic digestion of thermally pretreated wastewater solids (THD) was compared with conventional mesophilic anaerobic digestion (MAD). Operational conditions, such as pretreatment temperature (150 to 170°C), solids retention time (15 to 20 days), and digestion temperature (37 to 42°C), were varied for the seven THD systems operated. Volatile solids reduction (VSR) by THD ranged from 56 to 62%, compared with approximately 50% for MAD. Higher VSR contributed to 24 to 59% increased biogas production (m³/kg VSR·d) from THD relative to MAD. The high-solids conditions of the THD feed resulted in high total ammonia-nitrogen (proportional to solids loading) and total alkalinity concentrations in excess of 14 g/L as calcium carbonate (CaCO3). Increased pH in THD reactors caused 5 to 8 times more un-ionized ammonia to be present than in MAD, and this likely led to inhibition of aceticlastic methanogens, resulting in accumulation of residual volatile fatty acids between 2 and 6 g/L as acetic acid. The THD produced biosolids cake that possessed low organic sulfur-based biosolids odor and dewatered to between 33 and 39% total solids. Dual conditioning with cationic polymer and ferric chloride was shown to be an effective strategy for mitigating dissolved organic nitrogen and UV-quenching compounds in the return stream following centrifugal dewatering of THD biosolids. [ABSTRACT FROM AUTHOR]

The main objective of this research was to test the hypothesis that bioavailable protein and, more specifically, the sulfur-containing amino acids within the protein, can' be degraded by proteolytic enzymes to produce odor-causing compounds--mainly volatile sulfur compounds (VSCs)--during biosolids storage. To achieve these objectives, samples of digester effluent and cake solids were collected at 11 different wastewater treatment plants in North America, and the samples were analyzed for protein and amino acid content and general protein-degrading enzyme activity. At the same time, cake samples were stored using headspace bottles, the concentration of VSCs were measured using gas chromatography, and olfactometry measurements were made by a trained odor panel. The results showed that the bound cake protein content and methionine content was well-correlated with VSC production and the detection threshold measured by the odor panel. Water Environ. Res., 80, 127 (2008). [ABSTRACT FROM AUTHOR]

Abstract: A unique sludge digestion system consisting of anaerobic digestion followed by aerobic digestion and then a recycle step where thickened sludge from the aerobic digester was recirculated back to the anaerobic unit was studied to determine the impact on volatile solids (VS) reduction and nitrogen removal. It was found that the combined anaerobic/aerobic/anaerobic (ANA/AER/ANA) system provided 70% VS reduction compared to 50% for conventional mesophilic anaerobic digestion with a 20 day SRT and 62% for combined anaerobic/aerobic (ANA/AER) digestion with a 15 day anaerobic and a 5 day aerobic SRT. Total Kjeldahl nitrogen (TKN) removal for the ANA/AER/ANA system was 70% for sludge wasted from the aerobic unit and 43.7% when wasted from the anaerobic unit. TKN removal was 64.5% for the ANA/AER system. [ABSTRACT FROM AUTHOR]

Recent literature has reported that high concentrations of indicator bacteria such as fecal coliforms (FCs) were measured in anaerobically digested sludges immediately after dewatering even though low concentrations were measured prior to dewatering. This research hypothesized that the indicator bacteria can enter a non-culturable state during digestion, and are reactivated during centrifuge dewatering. Reactivation is defined as restoration of culturability. To examine this hypothesis, a quantitative polymerase chain reaction (qPCR) method was developed to enumerate Escherichia coli, a member of the FC group, during different phases of digestion and dewatering. For thermophilic digestion, the density of E. Coli measured by qPCR could be five orders of magnitude greater than the density measured by standard culturing methods (SCMs), which is indicative of non-culturable bacteria. For mesophilic digestion, qPCR enumerated up to about one order of magnitude more E. Coli than the SCMs. After centrifuge dewatering, the non-culturable organisms could be reactivated such that they are enumerated by SCMs, and the conditions in the cake allowed rapid growth of FCs and E. Coli during cake storage. [Copyright &y& Elsevier]

Laboratory anaerobic and aerobic digestion studies were conducted using waste activated sludges from two municipal wastewater treatment plants in order to gain insight into the mechanisms of floc destruction that account for changes in sludge conditioning and dewatering properties when sludges undergo anaerobic and aerobic digestion. Batch digestion studies were conducted at 20°C and the dewatering properties, solution biopolymer concentration and conditioning dose requirements measured. The data indicated that release of biopolymer from sludges occurred under both anaerobic and aerobic conditions but that the release was much greater under anaerobic conditions. In particular, the release of protein into solution was 4–5 times higher under anaerobic than under aerobic conditions. Both the dewatering rate, as characterized by the specific resistance to filtration and the amount of polymer conditioning chemicals required was found to depend directly on the amount of biopolymer (protein+polysaccharide) in solution. Little difference in dewatering properties and conditioning doses was seen between the two activated sludges from different plants. Differences in the cations released between anaerobic and aerobic digestion suggest that the digestion mechanisms differ for the two types of processes. Enyzme activity data showed that during aerobic digestion, polysaccharide degradation activity decreased to near zero and this was consistent with the accumulation of polysaccharides in aerobic digesters. [Copyright &y& Elsevier]

This study evaluated the impact of ammonia on mesophilic anaerobic digestion (AD) with thermal hydrolysis pretreatment (THP) treating a mixture of primary sludge and waste activated sludge and operated under constant organic loading rate of 9 kg COD/m3/d. Free ammonia concentrations in the digesters were varied between 37 and 966 mg NH3‐N/L, while maintaining all other operational conditions constant. A decrease in volatile solids reduction from 54 ± 5% (at <554 mg NH3‐N/L) to 35 ± 6% at the maximum free ammonia concentration of 966 mg NH3‐N/L was observed at steady‐state conditions. No impact of free ammonia on final dewaterability was detected. Free ammonia thus mostly limited methanogenesis. A free ammonia Monod inhibition constant of 847 ± 222 mg NH3‐N/L for methanogens was estimated based on the digester steady‐state methane rates dynamics. This study showed that current THP AD digesters (typically 110–260 mg NH3‐N/L) operate under 12%–18% ammonia inhibition for methanogenesis. Operation under SRT of 15 days, about 2 times more than needed to retain methanogens, can compensate for lower methanogens rates and avoid performance impacts. The later shows a good potential to operate under higher free and total ammonia concentration without jeopardizing performance. Practitioner points: Only from a free ammonia concentration above 554 mg NH3‐N/L, decreased volatile solids reduction and biogas yield were observed.A volatile solids reduction of 35 ± 6% at maximum free ammonia concentration of 966 mg NH3‐N/L was still achieved.A Monod inhibition constant for methanogens of 847 ± 222 mg NH3‐N/L was estimated.It was estimated that current THP AD systems (110–260 mg NH3‐N/L) operate under 12%–18% NH3 inhibition for methanogenesis. [ABSTRACT FROM AUTHOR]

This study evaluated the application of recuperative thickening (RT) to enhance anaerobic digestion (AD) performance for AD systems with thermal hydrolysis pretreatment (THP). RT was applied for two different reasons: (a) for increasing the sludge retention time (SRT) to degrade slowly hydrolyzable materials more efficiently and (b) for maintaining SRT at decreased hydraulic retention time (HRT) thus showing potential for increased AD throughput rates. A SRT increase from 15 to 30 days by RT application did not improve AD performance or hydrolysis rates significantly as 15‐day SRT was already a factor 2 higher than the estimated washout SRT. When applying RT to increase throughput rates (HRT of 7 days) while maintaining 15‐day SRT, no negative impact on biogas production or hydrolysis kinetics was observed. It was estimated that RT application on THP digesters can increase digester throughput by 100% and thus show clear potential for further AD intensification. Practitioner points: Increased SRT from 15 to 30 days through recuperative thickening application did not improve biogas production.A lower required minimum SRT (6–7 days) was estimated in THP‐AD systems compared to conventional AD.Operation at decreased HRT by RT application resulted in similar AD performance under constant organic loading rates.A 100% increase in throughput rates can be applied using RT without decreasing AD performance. [ABSTRACT FROM AUTHOR]

This study evaluated the role of bulk and substrate physics on hydrolysis rates and biogas yields in anaerobic digestion (AD) pretreated by thermal hydrolysis (THP). Although THP decreases sludge viscosity, no evidence was found that bulk viscosity impacted the biogas yield or hydrolysis kinetics. In addition, no significant difference between the biogas yields for different total solids concentrations nor floc sizes was detected. However, increased mixing speeds did increase biogas yields. As a result of thermal treatment, the model protein, bovine serum albumin, was harder to degrade in terms of both overall biodegradability and hydrolysis rates when their macrostructures were changed from liquid to gel and to solid structures; the opposite was true for the model polysaccharide, amylopectin. These results demonstrated that hydrolysis in THP‐AD systems was impacted mostly by the physical properties of the substrate (gelation) rather than the bulk physical properties within the digester. Practitioner points: Bulk viscosity does not significantly impact hydrolysis efficiency (biogas yield). However, mixing speed impacts hydrolysis beyond biogas holdup effect.Increasing the amount of substrate–microbe collisions through increasing biomass concentration does not have an impact on hydrolysis efficiency or biogas yield.Proteins are harder to degrade when macrostructure changes from liquid to gel/solid as a result of heat treatment.Polysaccharides are easier to degrade when macrostructure changes from liquid to gel/solid as a result of heat treatment.The time required for digesters to reach peak biogas production rates increased with decreasing specific surface available on gel and solid structures. [ABSTRACT FROM AUTHOR]

The goal of this study was to evaluate using feed dilution/solids retention time (SRT) control to manage potential ammonia inhibition in highly loaded anaerobic digesters after thermal hydrolysis. The study compared three digesters operated at the same target volatile solids (VS) loading rate of 5.5 kg VS/d‐m3, but at different feed concentrations resulting in SRTs of 10, 15, and 18 days. Lowering the feed concentration decreased the digester total ammonia nitrogen concentrations which averaged 1,580, 2,610, and 3,080 mg NH4+ ‐N/L for the 10‐, 15‐, and 18‐day digesters. The VS reduction and methane yields were equivalent for the 15‐ and 18‐day digesters and about 4% lower for the 10‐day digester. Ammonia inhibition of the 18‐day digester occurred early in the study, but the system acclimated over time. Feed dilution reduced the viscosity and the potential for volume expansion due to gas holdup and foaming. Practioner points: Feed dilution reduces digester ammonia concentrations and inhibition potential without sacrificing digester performance at lower SRTs.Feed dilution greatly reduces digester viscosity and associated issues with digester volume expansion due to gas holdup and foaming.Operating at the lower SRT does not impact cake solids after dewatering and substantially decreases polymer demand for conditioning. [ABSTRACT FROM AUTHOR]

Treatment of sewage sludge with a thermal hydrolysis process (THP) followed by anaerobic digestion (AD) enables to boost biogas production and minimize residual sludge volumes. However, the reject water can cause inhibition to aerobic and anoxic ammonium-oxidizing bacteria (AerAOB & AnAOB), the two key microbial groups involved in the deammonification process. Firstly, a detailed investigation elucidated the impact of different organic fractions present in THP-AD return liquor on AerAOB and AnAOB activity. For AnAOB, soluble compounds linked to THP conditions and AD performance caused the main inhibition. Direct inhibition by dissolved organics was also observed for AerAOB, but could be overcome by treating the filtrate with extended aerobic or anaerobic incubation or with activated carbon. AerAOB additionally suffered from particulate and colloidal organics limiting the diffusion of substrates. This was resolved by improving the dewatering process through an optimized flocculant polymer dose and/or addition of coagulant polymer to better capture the large colloidal fraction, especially in case of unstable AD performance. Secondly, a new inhibition model for AerAOB included diffusion-limiting compounds based on the porter-equation, and achieved the best fit with the experimental data, highlighting that AerAOB were highly sensitive to large colloids. Overall, this paper for the first time provides separate identification of organic fractions within THP-AD filtrate causing differential types of inhibition. Moreover, it highlights the combined effect of the performance of THP, AD and dewatering on the downstream autotrophic nitrogen removal kinetics. [ABSTRACT FROM AUTHOR]

A study was performed to evaluate the effect of thermal hydrolysis pretreatment (THP) temperature on subsequent digestion performance and operation, as well as downstream parameters such as dewatering and cake quality. A blend of primary and secondary solids from the Blue Plains treatment plant in Washington, DC was dewatered to about 16% total solids (TS), and thermally hydrolyzed at five different temperatures 130, 140, 150, 160, 170 °C. The thermally hydrolyzed solids were then fed to five separate, 10 L laboratory digesters using the same feed concentration, 10.5% TS and a solids retention time (SRT) of 15 days. The digesters were operated over a six month period to achieve steady state conditions. The higher thermal hydrolysis temperatures generally improved the solids reduction and methane yields by about 5–6% over the temperature range. The increased temperature reduced viscosity of the solids and increased the cake solids after dewatering. The dissolved organic nitrogen and UV absorbance generally increased at the higher THP temperatures. Overall, operating at a higher temperature improved performance with a tradeoff of higher dissolved organic nitrogen and UV adsorbing materials in the return liquor. [ABSTRACT FROM AUTHOR]

Abstract: Advanced anaerobic digestion processes aimed at improving the methanization of sewage sludge may be potentially impaired by the production of inhibitory compounds (e.g. free ammonia). The result of methanogenic inhibition is relatively high effluent concentrations of acetic acid and other soluble organics, as well as reduced methane yields. An extreme example of such an advanced process is the thermal hydrolytic pretreatment of sludge prior to high solids digestion (THD). Compared to a conventional mesophilic anaerobic digestion process (MAD), THD operates in a state of constant inhibition driven by high free ammonia concentrations, and elevated pH values. As such, previous investigations of the kinetics of methanogenesis from acetic acid under uninhibited conditions do not necessarily apply well to the modeling of extreme processes such as THD. By conducting batch ammonia toxicity assays using biomass from THD and MAD reactors, we compared the response of these communities over a broad range of ammonia inhibition. For both processes, increased inhibitor concentrations resulted in a reduction of biomass growth rate (r max = μ max∙X) and a resulting decrease in the substrate half saturation coefficient (K S ). These two parameters exhibited a high degree of correlation, suggesting that for a constant transport limited system, the K S was mostly a linear function of the growth rate. After correcting for reactor pH and temperature, we found that the THD and MAD biomass were both able to perform methanogenesis from acetate at high free ammonia concentrations (equivalent to 3–5 g/L total ammonia nitrogen), albeit at less than 30% of their respective maximum rates. The reduction in methane production was slightly less pronounced for the THD biomass than for MAD, suggesting that the long term exposure to ammonia had selected for a methanogenic pathway less dependent on those organisms most sensitive to ammonia inhibition (i.e. aceticlastic methanogens). [Copyright &y& Elsevier]

Abstract: The objective of this research was to investigate whether a preferential stimulation of microorganisms in anaerobically digested biosolids can occur after dewatering and if it can lead to pathogen indicator regrowth and odor generation upon storage. Laboratory incubation simulating biosolids storage indicates that both odorant generation, based on total volatile organic sulfur compound concentrations (TVOSCs) and pathogen indicator regrowth, based on fecal coliform densities follow similar formation and reduction patterns. The formation and reduction patterns of both odor compounds and fecal coliforms imply that groups of microorganism are induced if shearing disturbance is imposed during dewatering, but a secondary stabilization can be achieved soon after 1–2 weeks of storage. The occurrence of the induction is likely the microbial response to substrate release and environmental changes, such as oxygen, resulting from centrifuge shearing. The new conditions favor the growth of fecal coliforms and odor producing bacteria, and therefore, results in the observed fecal coliforms regrowth and odor accumulation during subsequent storage. However, when both substrate and oxygen deplete, a secondary stabilization can be achieved, and both odor and fecal coliforms density will drop. [Copyright &y& Elsevier]