Interaction between temperature and ammonia in mesophilic digesters for animal waste treatment

Four anaerobic sequencing batch reactors (ASBRs) were operated during a period of 988 days to evaluate the effect of temperature, ammonia, and their interconnectivity on the methane yield of anaerobic processes for animal waste treatment. During period 1 (day 0-378), the methane yield was 0.31 L CH₄/g volatile solids (VS) for all digesters (with no statistical differences among them) at a temperature and total ammonium-N levels of 25 °C and ∼1200 mg NH₄⁺-N/L, respectively. During period 2 (day 379-745), the methane yield at 25 °C decreased by 45% when total ammonium-N and ammonia-N were increased in two of the four ASBRs to levels >4000 mg NH₄⁺-N/L and >80 mg NH₃-N/L, respectively. During period 3 (day 746-988), this relative inhibition was reduced from 45% to 13% compared to the low-ammonia control reactors when the operating temperature was increased from 25 °C to 35 °C (while the free ammonia levels increased from ∼100 to ∼250 mg NH₃-N/L). The 10 °C increase in temperature doubled the rate constant for methanogenesis, which overwhelmed the elevated toxicity effects caused by the increasing concentration of free ammonia. Thus, the farmer/operator may alleviate ammonia toxicity by increasing the operating temperature within the mesophilic range. We extrapolated our data to correlate temperature, ammonia, and methane yield and to hypothesize that the difference between high- and low-ammonia reactors is negligible at the optimum mesophilic temperature of 38 °C.     

Reducing the environmental footprint of wastewater screenings through anaerobic digestion with resource recovery

Screenings produced as the first stage of wastewater treatment and currently disposed of to landfill, are rich in volatile organic solids, nitrogen and phosphorus which could be recovered through anaerobic digestion. Biochemical methane potential (BMP) tests on screenings demonstrated a methane yield of 0.33 m³ methane/kg volatile solids (VS) and a VS destruction of 50%. Consequently, the effect of a range of hydraulic retention time (HRT) and organic loading rates (OLRs) was evaluated in lab‐scale continuously fed mesophilic digesters. The highest methane yield of 0.416 Nm³ methane/kg VS added was observed with an HRT of 15 days and an OLR of 2.5 kg VS/m³/day, when up to 65% of the VS were destroyed. If treated by anaerobic digestion, every dry tonne of screenings digested would divert 466 kg from landfill, save 4.6 tonne equivalent carbon dioxide (CO_{2 eq}) and deliver 3.4 MWh of renewable energy.     

The anaerobic digestion of waste from an intensive pig unit

Anaerobic digestion of piggery wastes as a primary method of purification was studied in laboratory-scale (15 l.) heated, stirred digesters. It was impossible to obtain a balanced digestion by fermentating undiluted faeces-urine slurry, but balanced fermentations could be built up in digesters originally seeded from a working domestic anaerobic digester or in digesters filled with water to which small amounts of waste were regularly added. The results from running two digesters for over 80 weeks at loading rates from 0·03 to 0·20 lb VSS ft^?3 capacity day^?1 are given, and it is concluded that the anaerobic digestion of piggery wastes is possible and that improvement in the waste is obtained, but that there may be an upper limit to the solids content of the digester input. A secondary aerobic treatment of the settled anaerobic digester output improves the liquid.     

A new instrument for on-line measurement of bicarbonate alkalinity

Alkalinity or buffering in anaerobic digesters may be chiefly attributed to bicarbonate and should be at least 1000 mg CaCO₃/l for successful operation. A simple on-line instrument for the direct determination of bicarbonate concentration, especially for automatic control of anaerobic digesters, is described. The proposed method is based on a continuous measurement of the carbon dioxide flow rate evolved from a stream of sampled solution after saturation with gaseous CO₂ and subsequent acidification with excess acid. This eliminates the need for a pH probe which is subject to fouling.The device was calibrated using sodium bicarbonate solutions (0.01–0.065 M) and the calibration showed an accuracy of ±5%. The device was tested on effluent from anaerobic digesters operating on ice-cream waste supplemented with sodium bicarbonate within a range of 0.02–0.04 mol/l. The results compared favourably with titrimetric measurements, extensively used to determine bicarbonate alkalinity with an accuracy of ±5%. The response of the instrument to variation in alkalinity concentration is sufficiently rapid for most industrial applications.     

Performance improvement and decolourization in the high-solid thermophilic anaerobic digestion of thermally pretreated sewage sludge

This laboratory-scale study attempted performance improvement and decolourization in the high-solid thermophilic anaerobic digestion of thermally pretreated sewage sludge, as it tends to be disturbed by ammonia inhibition and colour generation. Sewage sludge was adjusted to 7%–8% total solids (TS), and pretreated at 150°C for 1 h. The digesters were operated at 55°C and 20 days hydraulic retention time. An addition of powdered activated carbon (approximately 2% of the feed TS) significantly contributed to the removal of propionate and reduced the colour in digested sludge by about 27%. Microbial analysis detected less abundance of bacterial Synergistia and archaeal Methanosarcina and implied more hydrogenotrophic methanogenesis with the activated carbon addition. Conditioning with ferric chloride for dewatering digested sludge mitigated the colour of dewatered liquor by about 67%. Therefore, these methods were demonstrated to be effective and partly overcome the above-mentioned problems.     

Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

The performance of thermophilic and mesophilic temperature co-phase anaerobic digestions for sewage sludge, using the exchange process of the digesting sludge between spatially separated mesophilic and thermophilic digesters, was examined, and compared to single-stage mesophilic and thermophilic anaerobic digestions. The reduction of volatile solids from the temperature co-phase anaerobic digestion system was dependent on the sludge exchange rate, but was 50.7-58.8%, which was much higher than 46.8% of single-stage thermophilic digestion, as well as 43.5% of the mesophilic digestion. The specific methane yield was 424-468 mL CH(4) per gram volatile solids removed, which was as good as that of single-stage mesophilic anaerobic digestion. The process stability and the effluent quality in terms of volatile fatty acids and soluble chemical oxygen demand of the temperature co-phase anaerobic digestion system were considerably better than those of the single-stage mesophilic anaerobic processes. The destruction of total coliform in the temperature co-phase system was 98.5-99.6%, which was similar to the single-stage thermophilic digestion. The higher performances on the volatile solid and pathogen reduction, and stable operation of the temperature co-phase anaerobic system might be attributable to the well-functioned thermophilic digester, sharing nutrients and intermediates for anaerobic microorganisms, and selection of higher substrate affinity anaerobic microorganisms in the co-phase system, as a result of the sludge exchange between the mesophilic and thermophilic digesters.     

The behaviour of nitrilotriacetic acid during the anaerobic digestion of sewage sludge

In laboratory simulations of the anaerobic sludge digestion process, the impact of the detergent builder nitrilotriacetic acid (NTA) on digester efficiency has been studied. It was concluded that NTA at concentrations up to 30 mg l⁻¹ had no adverse effect on anaerobic digestion. However, analysis of the digested sludge indicated that only 29–45% of the influent NTA was removed during treatment. To differentiate between biological and physical processes of removal, biological activity in the digesters was arrested by the addition of sodium azide. Effluent NTA concentration did not increase indicating that biological activity was not responsible for the removal observed.Batch experiments undertaken to evaluate NTA solubility in digested sludge (containing azide) at two solids concentrations indicated a decrease in soluble NTA with increasing solids concentration.It is concluded that the removal of NTA observed during anaerobic digestion was not biological and in part was the result of adsorption onto the solid phase.     

Bioaugmentation for improved recovery of anaerobic digesters after toxicant exposure

Bioaugmentation was investigated as a method to decrease the recovery period of anaerobic digesters exposed to a transient toxic event. Two sets of laboratory-scale digesters (SRT = 10 days, OLR = 2 g COD/L-day), started with inoculum from a digester stabilizing synthetic municipal wastewater solids (MW) and synthetic industrial wastewater (WW), respectively, were transiently exposed to the model toxicant, oxygen. Bioaugmented digesters received 1.2 g VSS/L-day of an H₂-utilizing culture for which the archaeal community was analyzed. Soon after oxygen exposure, the bioaugmented digesters produced 25-60% more methane than non-bioaugmented controls (p < 0.05). One set of digesters produced lingering high propionate concentrations, and bioaugmentation resulted in significantly shorter recovery periods. The second set of digesters did not display lingering propionate, and bioaugmented digesters recovered at the same time as non-bioaugmented controls. The difference in the effect of bioaugmentation on recovery may be due to differences between microbial communities of the digester inocula originally employed. In conclusion, bioaugmentation with an H₂-utilizing culture is a potential tool to decrease the recovery period, decrease propionate concentration, and increase biogas production of some anaerobic digesters after a toxic event. Digesters already containing rapidly adaptable microbial communities may not benefit from bioaugmentation, whereas other digesters with poorly adaptable microbial communities may benefit greatly.     

Volatile solids reduction in two-phase and conventional anaerobic sludge digestion

Success of two-phase anaerobic systems for primary and secondary sludge treatment has been reported based on both directly and indirectly measured volatile solids (VS) reduction, total gas and methane generation, COD reduction, etc. The objective of this research was to determine whether phase separation increases directly measured VS reduction compared to conventional anaerobic sludge digestion. Two-phase and conventional digesters were operated with sludge feeds from three sources; both 1:1 sludges (primary:waste activated, solids basis) and 100% waste activated sludges (WAS) were studied. The maximum difference between VS reductions in conventional and two-phase systems was about 8.7% with waste activated sludge. The increase in volatile solids reduction in two-phase systems with the 1:1 sludge ranged from 1.9 to 6.0% as compared to conventional systems. This relatively small increase in VS reduction may not be worth the additional cost of operating two-phase digesters at full scale.     

The effect of thermal hydrolysis pretreatment on the anaerobic degradation of nonylphenol and short-chain nonylphenol ethoxylates in digested biosolids

The presence of micropollutants can be a concern for land application of biosolids. Of particular interest are nonylphenol diethoxylate (NP₂EO), nonylphenol monoethoxylate (NP₁EO), and nonylphenol (NP), collectively referred to as NPE, which accumulate in anaerobically digested biosolids and are subject to regulation based on the environmental risks associated with them. Because biosolids are a valuable nutrient resource, it is essential that we understand how various treatment processes impact the fate of NPE in biosolids. Thermal hydrolysis (TH) coupled with mesophilic anaerobic digestion (MAD) is an advanced digestion process that destroys pathogens in biosolids and increases methane yields and volatile solids destruction. We investigated the impact of thermal hydrolysis pretreatment on the subsequent biodegradation of NPE in digested biosolids. Biosolids were treated with TH, anaerobic digestion, and aerobic digestion in laboratory-scale reactors, and NPE were analyzed in the influent and effluent of the digesters. NP₂EO and NP₁EO have been observed to degrade to the more estrogenic NP under anaerobic conditions; therefore, changes in the ratio of NP:NPE were of interest. The increase in NP:NPE following MAD was 56%; the average increase of this ratio in four sets of TH-MAD samples, however, was only 24.6 ± 3.1%. In addition, TH experiments performed in pure water verified that, during TH, the high temperature and pressure alone did not directly destroy NPE; TH experiments with NP added to sludge also showed that NP was not destroyed by the high temperature and pressure of TH when in a more complex sludge matrix. The post-aerobic digestion phases removed NPE, regardless of whether TH pretreatment occurred. This research indicates that changes in biosolids processing can have impacts beyond just gas production and solids destruction.     

Semi-continuous anaerobic digestion of solid poultry slaughterhouse waste: effect of hydraulic retention time and loading

We studied the effect of hydraulic retention time (HRT) and loading on anaerobic digestion of poultry slaughterhouse wastes, using semi-continuously fed, laboratory-scale digesters at 31 degrees C. The effect on process performance was highly significant: Anaerobic digestion appeared feasible with a loading of up to 0.8 kg volatile solids (VS)/m3 d and an HRT of 50-100 days. The specific methane yield was high, from 0.52 to 0.55 m3/kg VS(added). On the other hand, at a higher loading, in the range from 1.0 to 2.1 kg VS/m3 d, and a shorter HRT, in the range from 25 to 13 days, the process appeared inhibited and/or overloaded, as indicated by the accumulation of volatile fatty acids and long-chain fatty acids and the decline in the methane yield. However, the inhibition was reversible. The nitrogen in the feed, ca. 7.8% of total solids (TS), was organic nitrogen with little ammonia present, whereas in the digested material ammonia accounted for 52-67% (up to 3.8 g/l) of total nitrogen. The TS and VS removals amounted to 76% and 64%, respectively. Our results show that on a continuous basis under the studied conditions and with a loading of up to 0.8 kg VS/m3 d metric ton (wet weight) of the studied waste mixture could yield up to 140 m3 of methane.     

CFD simulation of gas and non-Newtonian fluid two-phase flow in anaerobic digesters

This paper presents an Eulerian multiphase flow model that characterizes gas mixing in anaerobic digesters. In the model development, liquid manure is assumed to be water or a non-Newtonian fluid that is dependent on total solids (TS) concentration. To establish the appropriate models for different TS levels, twelve turbulence models are evaluated by comparing the frictional pressure drops of gas and non-Newtonian fluid two-phase flow in a horizontal pipe obtained from computational fluid dynamics (CFD) with those from a correlation analysis. The commercial CFD software, Fluent12.0, is employed to simulate the multiphase flow in the digesters. The simulation results in a small-sized digester are validated against the experimental data from literature. Comparison of two gas mixing designs in a medium-sized digester demonstrates that mixing intensity is insensitive to the TS in confined gas mixing, whereas there are significant decreases with increases of TS in unconfined gas mixing. Moreover, comparison of three mixing methods indicates that gas mixing is more efficient than mixing by pumped circulation while it is less efficient than mechanical mixing.     

Pre-treatment mechanisms during thermophilic-mesophilic temperature phased anaerobic digestion of primary sludge

Pre-treatment is used extensively to improve degradability and hydrolysis rate of material being fed into digesters. One emerging process is temperature phased anaerobic digestion (TPAD), which applies a short (2 day) 50-70 °C pre-treatment step prior to 35 °C digestion in the main stage (10-20 days). In this study, we evaluated a thermophilic-mesophilic TPAD against a mesophilic-mesophilic TPAD treating primary sludge. Thermophilic-mesophilic TPAD achieved 54% VS destruction compared to 44% in mesophilic-mesophilic TPAD, with a 25% parallel increase in methane production. Measurements of soluble COD and NH₄⁺-N showed increased hydrolysis extent during thermophilic pre-treatment. Model based analysis indicated the improved performance was due to an increased hydrolysis coefficient rather than an increased inherent degradability, suggesting while TPAD is suitable as an intensification process, a larger main digester could achieve similar impact.     

CONTROLLING AND MONITORING ANAEROBIC DIGESTERS FED WITH THERMALLY HYDROLYSED SLUDGE

A sludge treatment centre has been constructed at Nigg sewage-treatment works, Aberdeen to receive sewage sludge from a number of plants in the area. The sludge is treated to the USEPA Class A standard in a thermal-hydrolysis plant in which it is heated to 165°C for 30 mins in batch reactors. The hydrolysed sludge, which contains 10–12% dry solids, is then pumped to an anaerobic digestion plant where the temperature is maintained at 39°C for a minimum of 15 days.
The commissioning and testing of the anaerobic digesters was different to those which are fed with untreated sludge, and the approach is outlined. The paper discusses (a) the digestion parameters which were monitored during start-up, (b) the performance of the digesters throughout the commissioning and testing period, and (c) the quality of the dewatered sludge.     

Pentachlorophenol biodegradation—II : Anaerobic

The fate of pentachlorophenol (PCP) during anaerobic digestion of sewage sludge solids was tested in a three phase protocol. Phase I involved acclimation; Phase II investigated biodegradation in semicontinuous-flow, stirred-tank reactors at solids retention times of 10, 20 and 40 days; phase III assessed the importance of nonbiological removal mechanisms and collected additional data concerning the extent of biodegradation. PCP was found to inhibit methanogenesis in unacclimated cultures at concentrations in excess of 200 μg l⁻¹ and thus acclimation of the digesters to PCP required very gradual increases in the influent concentration thereby allowing enrichment of organisms capable of degrading PCP. Once acclimation was achieved, digesters receiving influent containing 5.0 mg l⁻¹ PCP achieved stable operation with effluent PCP concentrations below 5 μg l⁻¹ at all retention times studied. Sorption was shown to be unimportant in PCP removal. Volatilization was considered to be unimportant based upon results obtained in forced aeration studies. This suggested that PCP was subject to at least primary biodegradation. Other evidence collected during the study indicated that more complete biodegradation probably was occurring.     

CFD simulation of mixing in egg-shaped anaerobic digesters

A computational fluid dynamics (CFD) model that characterizes mechanical draft tube mixing in egg-shaped anaerobic digesters was developed. Simulation of flow patterns were carried out with a propeller rotating from 400 to 750 rpm, assuming liquid manure to be Newtonian (water) and non-Newtonian fluids depending on the total solids (TS) concentration. Power number and flow number of the propeller in water mixing were validated against lab specifications and experimental data from a field test. The rotational direction and placement of the propeller were examined to identify the primary pumping mode and the optimum position of the propeller fixed inside the tube. Quantitative comparisons of two mixing methods and two digester shapes indicated that mechanical draft tube mixing is more efficient than external pumped recirculation, and that the egg shape provides for more efficient mixing than the cylindrical shape. Furthermore, scale-up rules for mixing in egg-shaped digesters were investigated.     

Anaerobic digestion of animal waste: effect of mode of mixing

Laboratory-scale digesters were operated to study the effect of mixing (via biogas recirculation, impeller mixing, and slurry recirculation) on biogas production. Three sets of experiments were performed using cow manure slurry feed with either 50, 100, or 150 g/L total solids (TS) concentrations (referred in the text as 5%, 10%, and 15% manure slurry). The experiments were conducted at a controlled temperature of 35 degrees C and a hydraulic retention time of 16.2 days, resulting in TS loadings of 3.1, 6.2, and 9.3g/Ld for 5%, 10%, and 15% manure slurry feeds, respectively. Results showed that the unmixed and mixed digesters performed quite similarly when fed with 5% manure slurry and produced biogas at a rate of 0.84-0.94 L/Ld. The methane yield was found to be 0.26-0.28 L CH4/g volatile solids loaded. However, the effect of mixing and the mode of mixing became important when the digesters were fed thick manure slurry feeds (10% and 15%). Digesters fed with 10% and 15% manure slurry and equipped with external mixing produced about 10-30% more biogas than the unmixed digester. While the mixed digesters produced more biogas than unmixed digesters, digester mixing during start-up was not beneficial, as it resulted in lower pH, performance instability and prolonged start-up time. Mixing using biogas recirculation system was found not to be effective in the case of 15% manure slurry feed under the experimental conditions studied.     

Methanogen community structure-activity relationship and bioaugmentation of overloaded anaerobic digesters

Accumulation of acids in anaerobic digesters after organic overload can inhibit or stop CH₄ production. Therefore, methods to reduce acid concentrations would be helpful. One potential method to improve recovery involves bioaugmentation, addition of specific microorganisms to improve performance. In this study, transiently overloaded digesters were bioaugmented with a propionate-degrading enrichment culture in an effort to decrease recovery time. Biomass samples from 14 different, full-scale anaerobic digesters were screened for specific methanogenic activity (SMA) against propionate; the microbial communities were also compared. SMA values spanned two orders of magnitude. Principal component analysis of denaturing gradient gel electrophoresis (DGGE) banding patterns for a functional gene (mcrA) suggested an underlying community structure-activity relationship; the presence of hydrogenotrophic methanogens closely related to Methanospirillum hungatei and Methanobacterium beijingense was associated with high propionate SMA values. The biomass sample demonstrating the highest SMA was enriched for propionate degrading activity and then used to bioaugment overloaded digesters. Bioaugmented digesters recovered more rapidly following the organic overload, requiring approximately 25 days (2.5 solids retention times (SRTs)) less to recover compared to non-bioaugmented digesters. Benefits of bioaugmentation continued for more than 12 SRTs after organic overload. Bioaugmentation is a promising approach to decrease recovery time after organic overload.     

Potential effects of desalinated water quality on the operation stability of wastewater treatment plants

Desalinated water is expected to become the major source of drinking water in many places in the near future, and thus the major source of wastewater to arrive at wastewater treatment plants. The paper examines the effect of the alkalinity value with which the water is released from the desalination plant on the alkalinity value that would develop within the wastewater treatment process under various nitrification-denitrification operational scenarios. The main hypothesis was that the difference in the alkalinity value between tap water and domestic wastewater is almost exclusively a result of the hydrolysis of urea (NH₂CONH₂, excreted in the human urine) to ammonia (NH₃), regardless of the question what fraction of NH_3(aq) is transformed to NH₄⁺. Results from a field study show that the ratio between the alkalinity added to tap water when raw wastewater is formed (in meq/l units) and the TAN (total ammonia nitrogen, mole/l) concentration in the raw wastewater is almost 1:1 in purely domestic sewage and close to 1:1 in domestic wastewater streams mixed with light industry wastewaters. Having established the relationship between TAN and total alkalinity in raw wastewater the paper examines three theoretical nitrification-denitrification treatment scenarios in the wastewater treatment plant (WWTP). The conclusion is that if low-alkalinity desalinated water constitutes the major water source arriving at the WWTP, external alkalinity will have to be added in order to avoid pH drop and maintain process stability. The results lead to the conclusion that supplying desalinated water with a high alkalinity value (e.g. ≥ 100 mg/l as CaCO₃) would likely prevent the need to add costly basic chemicals in the WWTP, while, in addition, it would improve the chemical and biological stability of the drinking water in the distribution system.