The role of HRT and low temperature on the acid-phase anaerobic digestion of municipal and industrial wastewaters

Two identical 31 completely mixed reactors with solids recycling capabilities were used to investigate the effects of hydraulic retention time (HRT) and low temperatures on volatile fatty acid (VFA) production. One reactor was fed with a 1:1 ratio of diluted primary sludge and a starch-rich industrial wastewater, while the other was fed with diluted primary sludge alone. The VFA and soluble COD concentrations and specific production rates reached their highest values at 30 h HRT and at 25 degrees C. Further increase in HRT (at 25 degrees C) or decrease in temperature (at an HRT of 30 h) resulted in lower amounts of VFA and COD produced. All parameters related to VFA and COD production were significantly higher in the industrial-municipal reactor than in the municipal-only reactor. The VFA:COD ratios were very high, with values ranging from about 0.8 to 1.0 indicating that hydrolysis was the rate-limiting step. Degradation of proteins (measured by ammonia production) was inhibited by the starch-rich wastewater in the industrial-municipal reactor, while no evidence of inhibition was found in the municipal-only reactor. This study revealed that VFA production was feasible at low temperatures (down to 8 degrees C), particularly in the presence of the industrial waste. Ultimately, the amount VFA produced was adequate, in most cases, to support subsequent biological nutrient removal (BNR) processes.     

Production of volatile fatty acids by fermentation of waste activated sludge pre-treated in full-scale thermal hydrolysis plants

This work focuses on fermentation of pre-treated waste activated sludge (WAS) to generate volatile fatty acids (VFAs). Pre-treatment by high-pressure thermal hydrolysis (HPTH) was shown to aid WAS fermentation. Compared to fermentation of raw WAS, pre-treatment enabled a 2-5x increase in VFA yield (gVFA(COD)gTCOD(-1)) and 4-6x increase in VFA production rate (gVFA(COD) L(-1) d(-1)). Three sludges, pre-treated in full-scale HPTH plants, were fermented. One was from a plant processing a mix of primary sludge and WAS and the other two from plants processing solely WAS. The HPTH plants solubilised suspended matter, evidenced by a 20-30% decrease in suspended solids and an increase of soluble COD : total COD from 0.04 to 0.4. Fermentation of the three sludges yielded similar VFA concentrations (15-20gVFA(COD) L(-1)). The yields were largely independent of retention time (1 d-6 d) and temperature (42°C, 55°C). Also, the product spectrum depended mostly on the composition of the sludge rather than on operating conditions.     

The influence of anaerobic pretreatment on the nitrogen removal from biosynthetic pharmaceutical wastewaters

The C:N ratio of the pharmaceutical wastewaters is usually suitable for a combination of the anaerobic pretreatment with the high COD removal and aerobic posttreatment with the efficient biological N removal. This kind of anaerobic-aerobic process was tested in semipilot scale by using a UASB reactor and an activated sludge system with a predenitrification (total volume 100 1). It was found that at a total HRT of 2.3 days an average of 97.5% of COD and 73.5% of total N was removed. The UASB reactor was operated at 30°C with a volumetric loading rate of 8.7 kg.m^-3.d^-1, the efficiency of COD removal was 92.2%. The processes, which take part in the biological removal of nitrogen, especially the nitrification, were running with lower rates than usually observed in aerobic treatment systems.Abbreviations AAO anaerobic anoxic oxic configuration - AOO anaerobic oxic oxic configuration - B _V volumetric organic loading rate (kg COD.m^-3. d^-1) - dB _x specific COD removal rate (mg COD. g^-1 VSS. d^-1) - DNR denitrification rate (mg N–NO₃. g^-1 VSS. h^-1) - E_COD efficiency of COD removal (%) - HRT hydraulic retention time (d) - NR nitrification rate (mg N–NO₃. g^-1 VSS. h^-1) - R recirculation ratio (%) - SBP specific biogas production (m³.kg^-1 removed COD) - SRT solids retention time; sludge age (d) - SS suspended solids (g.1^-1) - UASB upflow anaerobic sludge blanket reactor - VSS volatile suspended solids (g.1^-1)     

Low temperature thermo-chemical pretreatment of dairy waste activated sludge for anaerobic digestion process

An investigation into the influence of low temperature thermo-chemical pretreatment on sludge reduction in a semi-continuous anaerobic reactor was performed. Firstly, effect of sludge pretreatment was evaluated by COD solubilization, suspended solids reduction and biogas production. At optimized condition (60 °C with pH 12), COD solubilization, suspended solids, reduction and biogas production was 23%, 22% and 51% higher than the control, respectively. Secondly, semi-continuous process performance was studied in a lab-scale semi-continuous anaerobic reactor (5 L), with 4 L working volume. With three operated SRTs, the SRT of 15 days was found to be most appropriate for economic operation of the reactor. Combining pretreatment with anaerobic digestion led to 80.5%, 117% and 90.4% of TS, SS and VS reduction respectively, with an improvement of 103% in biogas production. Thus, low temperature thermo-chemical can play an important role in reducing sludge production.     

Anaerobic digestion of grass silage in batch leach bed processes for methane production

Anaerobic digestion of grass silage in batch leach bed reactors, with and without a second stage upflow anaerobic sludge blanket (UASB) reactor, was evaluated. Sixty six percent of the methane potential in grass was obtained within the 55 days solids retention time in the leach bed-UASB process without pH adjustment, whereas in the one-stage leach bed process 20% of the methane potential in grass was extracted. In two-stage operation, adjustment of the pH of influent to the leach bed reactor to 6 with HCl led to inhibition of both hydrolysis/acidogenesis and methanogenesis. In the leach bed-UASB process 39% of the carbohydrates and 58% of the acid soluble lignin were solubilised within the 49 days of operation, whereas Klason lignin was most recalcitrant. The methane potential of the digestates varied from 0.141 to 0.204 m3 CH4 kg(-1) added volatile solids.     

Anaerobic membrane reactor with phase separation for the treatment of cheese whey

Two-phase anaerobic digestion of cheese whey was investigated in a system consisting of a stirred acidogenic reactor followed by a stirred methanogenic reactor, the latter being coupled to a membrane filtration system to enable removal of soluble effluent whilst retaining solids. The acidogenic reactor was operated at a hydraulic retention time (HRT) of one day, giving maximum acidification of 52.25% with up to 5 g/l volatile fatty acids, of which 63.7% was acetic acid and 24.7% was propionic acid. The methanogenic reactor received an organic load up to 19.78 g COD/ld, corresponding to a HRT of 4 days, at which 79% CODs and 83% BOD(5) removal efficiencies were obtained. Average removals of COD, BOD(5) and TSS in the two-phase anaerobic digestion process were 98.5%, 99% and 100%, respectively. The daily biogas production exceeded 10 times reactor volume and biogas methane content was greater than 70%.     

Production of biogas from water hyacinth (Eichhornia crassipes) (Mart) (Solms) in a two-stage bioreactor

A two-stage rumen-derived anaerobic digestion process was tested for the conversion of water hyacinth shoots and a mixture of the shoots with cowdung (7:3) into biogas. Under conditions similar to those of the rumen and loading rates (LR) in the range of 11.6–19.3g volatile solids (VS) l–1d–1 in the rumen reactor, the degradation efficiencies were 38% for the shoots and 43% for the mixture. The major fermentation products were volatile fatty acids (VFA) with a maximum yield of 7.92mmolg–1 VS digested, and biogas with a yield of 0.2lg–1 VS digested. The effect of varying LR, solid retention time (SRT) and dilution rates on the extent of degradation of the water hyacinth–cowdung mixture was examined. Overall conversion of the substrate was highest at the loading rate of 15.4gVS.l–1d–1. Varying the retention times between 60 and 120h had no effect on the degradation efficiency, but a decrease was observed at retention times below 60h. The overall performance of the reactor was depressed by changing the dilution rate from 0.5 to 0.34h–1. By applying a LR of 15.4VS. l–1d–1, a SRT of 90h and a dilution rate of 0.5h–1 in the rumen reactor, and connecting it to a methanogenic reactor of the upflow anaerobic sludge blanket type, 100% conversion efficiency of the VFA into biogas with a methane content of 80% was achieved. The average methane gas yield was 0.44lg–1 VS digested.     

Mesophilic anaerobic treatment of sludge from salmon smolt hatching

The mesophilic anaerobic treatment of concentrated sludge from an Atlantic salmon smolt hatchery (total solids (TS): 6.3-12.3wt%) was investigated in a continuous stirred tank reactor (CSTR) at 35 degrees C and 55-60 days hydraulic retention time (HRT). COD-stabilization between 44% and 54% and methane yields between 0.140 and 0.154l/g COD added (0.260-0.281l/g VS added) were achieved. The process was strongly inhibited, with volatile fatty acid concentrations of up to 28 g/l. But the buffer capacity was sufficient to keep the pH-value at 7.4-7.55 during the whole operation. The fertilizing value of the treated sludge was estimated to be 3.4-6.8 kg N and 1.2-2.4 kg P per ton. However, the high VFA content would necessitate special means of application. The energy from the methane that was achieved in the present study would be sufficient to cover about 2-4% of the energy demands of a flow-through hatchery.     

Effect of feed composition and upflow velocity on aggregate characteristics in anaerobic upflow reactors

Two upflow anaerobic hybrid reactors treated lactose and a mixture of ethanol, propionate and butyrate, respectively, at a volumetric loading rate of 3.7 kg chemical oxygen demand (COD) m⁻³day⁻¹, a hydraulic retention time of 5 days and a liquid upflow velocity of 0.01 m/h. Under steady-state conditions, the lactose-fed sludge had much higher (20%–100%) specific methanogenic conversion rates than the volatile-fatty acid␣(VFA)/ethanol-fed sludge for all substrates tested, including VFA. In both reactors, a flocculant sludge developed, although a much higher content of extracellular polysaccharide was measured in the lactose-fed sludge [1900 μg compared to 305 μg uronic acid/g volatile suspended solids (VSS)]. When the liquid upflow velocity of a third, VFA/ethanol-fed reactor was increased to 0.5 m/h, granulation of the sludge occurred, accompanied by a large increase (200%–500%) in the specific methanogenic conversion rates for the syntrophic and methanogenic substrates studied. Granulation reduced the susceptibility of the sludge to flotation. Glucose was degraded at a high rate (100 mg glucose gVSS⁻¹h⁻¹) by the sludge from the third reactor, despite not having been exposed to a sugar-containing influent for 563␣days. Received: 7 June 1996 / Received revision: 23 September 1996 / Accepted: 29 September 1996     

Start-up of an anaerobic hybrid (UASB/filter) reactor treating wastewater from a coffee processing plant

The ability of an anaerobic hybrid reactor, treating coffee wastewater, to achieve a quick start-up was tested at pilot scale. The unacclimatized seed sludge used showed a low specific methanogenic activity of 26.47 g CH4 as chemical oxygen demand (COD)/kg volatile suspended solids (VSS) x day. This strongly limited the reactor performance. After a few days of operation, a COD removal of 77.2% was obtained at an organic loading rate (OLR) of 1.89 kg COD/m3 x day and a hydraulic retention time (HRT) of 22 h. However, suddenly increasing OLR above 2.4 kg COD/m3 x day resulted in a deterioration in treatment efficiency. The reactor recovered from shock loads after shutdowns of 1 week. The hybrid design of the anaerobic reactor prevented the biomass from washing-out but gas clogging in the packing material was also observed. Wide variations in wastewater strength and flow rates prevented stable reactor operation in the short period of the study.     

Ambient temperature treatment of low strength wastewater using anaerobic sequencing batch reactor

Low strength wastewater having chemical oxygen demands (COD) concentrations of 1000, 800, 600 and 400mg/l were treated at 35, 25, 20 and 15¡C using four anaerobic sequencing batch reactors (ASBRs). Reactor 1 was operated at hydraulic retention time (HRT) of 48h, reactor 2 at 24h HRT, reactor 3 at 16h HRT and reactor 4 at 12h HRT. 80 to 99% soluble COD was removed at the various operational conditions, except during 15¡C treatment of 1000 and 800mg/l COD wastewater at 12h HRT and 1000mg/l COD wastewater at 16h HRT, where excessive loss of biological solids occurred. The ASBR process can be an effective process for the treatment of low concentrated wastewaters which are usually treated aerobically with large amount of sludge production and higher energy expenditures.     

Mesophilic anaerobic digestion in a fluidised-bed reactor of wastewater from the production of protein isolates from chickpea flour

A study of the anaerobic digestion of wastewater derived from the production of protein isolates from chickpea flour was carried out in a laboratory-scale, mesophilic (35 °C) fluidised-bed reactor with saponite as bacterial support. Soluble chemical oxygen demand (SCOD) removal efficiencies in the range of 96.8–85.2% were achieved in the reactor at organic loading rates (OLR) of between 0.58 and 2.10 g chemical oxygen demand (COD)/l per day, hydraulic retention times (HRT) of between 14.9 and 4.5 days and average feed COD concentration of 9.1 g/l. Eighty-five percent of feed COD could be removed up to OLR of 2.1 g COD/l per day. The yield coefficient of methane production was 0.34 l of methane (at STP) per gram COD removed and was virtually independent of the OLR applied. Because the buffering capacity of the experimental system was maintained at favourable levels with excess total alkalinity present at all loadings, the rate of methanogenesis was not affected by loading. Experimental data indicated that a total alkalinity in the range of 1090–2130 mg/l as CaCO₃ was sufficient to prevent the pH from decreasing to below 7.2 for OLR of up to 2.7 g COD/l per day. The volatile fatty acid (VFA) levels and the VFA/alkalinity ratio were lower than the suggested limits for digester failure (0.3–0.4) for OLR and HRT up to 2.7 g COD/l per day and 3.5 days, respectively. For a HRT of 2.8 days (OLR of 3.00 g COD/l per day) the start of acidification was observed in the reactor.     

Ammonia–methane two-stage anaerobic digestion of dehydrated waste-activated sludge

The study investigated methane production from dehydrated waste-activated sludge (DWAS) with approximately 80% water content under thermophilic conditions. The repeated batch-wise treatment of DWAS using methanogenic sludge unacclimated to high concentrations of ammonia, increased the ammonia production up to 7,600 mg N per kilogram total wet sludge of total ammonia concentration, and stopped the methane production. Investigation revealed that the loading ratio of DWAS for methanogenic sludge influences anaerobic digestion. Methane production significantly decreased and ammonia concentration increased with the increase in loading ratio of DWAS. Since the semicontinuous culture revealed that approximately 50% of organic nitrogen in DWAS converted to ammonia at sludge retention time (SRT) after 4 days at 37 degrees C and 1.33 days at 55 degrees C, the previous stripping of the ammonia produced from DWAS was carried out. The stripping of ammonia increased methane production significantly. This ammonia-methane two-stage anaerobic digestion demonstrated a successful methane production at SRT 20 days in the semicontinuous operation using a laboratory-scale reactor system.     

Ultrasonic sludge disintegration for enhanced methane production in anaerobic digestion: effects of sludge hydrolysis efficiency and hydraulic retention time

Hydrolysis of waste activated sludge (WAS) has been regarded as the rate limiting step of anaerobic sludge digestion. Therefore, in this study, the effect of ultrasound and hydraulic residence time during sludge hydrolysis was investigated with the goal of enhancing methane production from anaerobic digestion (AD). WAS was ultrasonically disintegrated for hydrolysis, and it was semi-continuously fed to an anaerobic digesters at various hydraulic retention times (HRTs). The results of these experiments showed that the solids and chemical oxygen demand (COD) removal efficiencies when using ultrasonically disintegrated sludge were higher during AD than the control sludge. The longer the HRT, the higher the removal efficiencies of solids and COD, while methane production increased with lower HRT. Sludge with 30% hydrolysis produced 7 × more methane production than the control sludge. The highest methane yields were 0.350 m(3)/kg volatile solids (VS)(add) and 0.301 m(3)/kg COD(con) for 16 and 30% hydrolyzed sludge, respectively. In addition, we found that excess ultrasound irradiation may inhibit AD since the 50% hydrolyzed sludge produced lower methane yields than 16 and 30% hydrolyzed sludge.     

Performances of a full-scale novel multiplate anaerobic reactor treating cheese whey effluent

A 450-m(3) multiplate anaerobic reactor (MPAR) has been started-up in April 1992 for treating wastewater (whey permeate and domestic wastewater) at the Nutrinor (Lactel) cheese factory in Chambord (Québec, Canada). The MPAR consists of four superimposed sections. The liquid flows upwards from one section to the next, while the gas is collected below each plate and evacuated through side-outlets. The wastewater is concurrently distributed at the bottom of the first, second, and third sections, as 50%, 33%, and 17% of the total influent stream, respectively. Granular anaerobic sludge at an initial concentration of 30 kg of volatile suspended solids (VSS) per cubic meter of reactor liquid volume was used to inoculate the reactor. Under normal operation of the factory, the chemical oxygen demand (COD) concentration of the influent ranged from 20 to 37 kg COD m(-3). The reactor organic loading rate (OLR) fluctuated between 9 and 14.7 kg COD m(-3) d(-1) for hydraulic retention times (HRT) maintained between 55 and 68 h. At the highest OLR, the MPAR showed an efficiency of 98% and 92% for soluble and total COD removal, respectively, and a methane production rate averaging around 4 m(3) m(-3) d(-1).Biomass-specific activities ranged between 7 and 51, 1.3 and 8.5, 5.3 and 12.2, 60 and 119, and 119 and 211 mmol g(-1) VSS d(-1) for glucose, propionate, acetate, formate, and hydrogen, respectively. Average equivalent-diameter of the granules was around 0.65 mm. The MPAR reactor generally showed a large capacity for solid retention with a biomass content between 32 and 37 kg VSS m(-3). (c) 1995 John Wiley & Sons, Inc.     

Mesophilic anaerobic treatment of sludge from saline fish farm effluents with biogas production

The mesophilic anaerobic treatment of sludge from saline fish farm effluents (total solids (TS): 8.2-10.2 wt%, chemical oxygen demand (COD): 60-74 g/l, sodium (Na): 10-10.5 g/l) was carried out in continuously stirred tank reactors (CSTRs) at 35 degrees C. COD stabilization between 36% and 55% and methane yields between 0.114 and 0.184 l/g COD added were achieved. However, the process was strongly inhibited, presumably by sodium, and unstable, with propionic acid being the main compound of the volatile fatty acids (VFA). When diluting the sludge 1:1 with tap water (Na: 5.3 g/l), the inhibition could be overcome and a stable process with low VFA concentrations was achieved. The results of the study are used to make recommendations for the configuration of full-scale treatment plants for the collected sludge from one salmon farming licence and to estimate the energy production from these plants.     

A further study of the anaerobic biotreatment of malt whisky distillery pot ale using an UASB system

Pot ale from a pilot-scale malt whisky distillery was treated using a mesophilic upflow anaerobic sludge blanket (UASB) digester. Stable operation was observed at organic loading rates (OLRs) of 5.46 kg COD/m3 day or less when the pot ale was diluted with tap water. Digester failure occurred when undiluted pot ale was used, even though OLR was less than 5 kg COD/m3 day. Overall performance was worse than that observed previously when UASB digesters were used to treat pot ale from a different source supplemented with trace elements. A substantial proportion of effluent chemical oxygen demand (COD) was present as volatile fatty acids (VFA), particularly during periods of reactor stress, indicating that overall performance was limited by the rate of VFA conversion. Wastewater alkalinity rose during digestion. The sludge which developed in the reactor was flocculent but did not form compact granules.     

Performance evaluation of a mesophilic anaerobic fluidized-bed reactor treating wastewater derived from the production of proteins from extracted sunflower flour

A study of the anaerobic digestion of wastewater derived from the production of protein isolates from extracted sunflower flour was carried out in a laboratory-scale, mesophilic (35 degrees C) fluidized-bed reactor with saponite as bacterial support. Chemical oxygen demand (COD) removal efficiencies in the range of 98.3-80.0% were achieved in the reactor at organic loading rates (OLR) of between 0.6 and 9.3 g COD/I d, hydraulic retention times (HRT) of between 20.0 and 1.1 d and average feed COD concentration of 10.6 g/l. Eighty percent of feed COD could be removed up to OLR of 9.3 g COD/l d. The yield coefficient of methane production was 0.33 l of methane (at STP) per gram of COD removed and was virtually independent of the OLR applied. Because the buffering capacity of the experimental system was maintained at favorable levels with excess total alkalinity present at all loadings, the rate of methanogenesis was not affected by loading. The experimental data indicated that a total alkalinity in the range of 2,000-2,460 mg/l as CaCO3 was sufficient to prevent the pH from dropping to below 7.0 for OLR of up to 9.3 g COD/l d. The volatile fatty acid (VFA) levels and the VFA/alkalinity ratio were lower than the suggested limits for digester failure (0.3-0.4) for OLR and HRT up to 9.3 g COD/l d and 1.1 d, respectively. For a HRT of 0.87 d (OLR of 12.1 g COD/l d) the start of acidification was observed in the reactor.     

Long-term competition between sulfate reducing and methanogenic bacteria in UASB reactors treating volatile fatty acids

The competition between acetate utilizing methane-producing bacteria (MB) and sulfate-reducing bacteria (SRB) was studied in mesophilic (30 degrees C) upflow anaerobic sludge bed (UASB) reactors (upward velocity 1 m h-1; pH 8) treating volatile fatty acids and sulfate. The UASB reactors treated a VFA mixture (with an acetate:propionate:butyrate ratio of 5:3:2 on COD basis) or acetate as the sole substrate at different COD:sulfate ratios. The outcome of the competition was evaluated in terms of conversion rates and specific methanogenic and sulfidogenic activities. The COD:sulfate ratio was a key factor in the partitioning of acetate utilization between MB and SRB. In excess of sulfate (COD:sulfate ratio lower than 0.67), SRB became predominant over MB after prolonged reactor operation: 250 and 400 days were required to increase the amount of acetate used by SRB from 50 to 90% in the reactor treating, respectively, the VFA mixture or acetate as the sole substrate. The competition for acetate was further studied by dynamic simulations using a mathematical model based on the Monod kinetic parameters of acetate utilizing SRB and MB. The simulations confirmed the long term nature of the competition between these acetotrophs. A high reactor pH (+/-8), a short solid retention time (<150 days), and the presence of a substantial SRB population in the inoculum may considerably reduce the time required for acetate-utilising SRB to outcompete MB.     

Development of thermophilic methanogenic sludge in compartmentalized upflow reactors

The characteristics and development of thermophilic anaerobic sludge in upflow staged sludge bed (USSB) reactors were studied. The compartmentalized reactors were inoculated with partially crushed mesophilic granular sludge and then fed with either a mixture of volatile fatty acids (VFA) or a mixture of sucrose and VFA. The staged degradation of the soluble substrate in the various compartments led to a clear segregation of specific types of biomass along the height of the reactor, particularly in reactors fed with the sucrose-VFA mixture. Both the biological as well as the physical properties of the cultivated sludge were affected by the fraction of nonacidified substrate. The sludge in the first compartment of the reactor treating the sucrose-VFA mixture was whitish and fluffy, most likely resulting from the development of acidifying bacteria. Sludge granules which developed in the top part of this reactor possessed the highest acetogenic and methanogenic activity and the highest granule strength as well. The experiments also revealed that the conversion of the sucrose-VFA mixture into methane gradually deteriorated at prolonged operation at high organic loading rates (50 to 100 g COD . L(-1) . day(-1)). Stable long-term performance of a reactor can only be achieved by preserving the sludge segregation along the height of the reactor. In the reactor fed solely with the VFA mixture little formation of granular sludge occurred. In this reactor, large differences in sludge characteristics were also observed along the reactor height. Li(+)-tracer experiments indicated that the hydraulic regime in the USSB reactor is best characterized by a series of at least five completely mixed reactors. The formation of granular sludge was found to influence the liquid flow pattern. (c) 1996 John Wiley & Sons, Inc.