Anaerobic acidogenesis of dairy wastewater: the effects of variations in hydraulic retention time with no pH control

The performance of a laboratory‐scale mesophilic acidogenic reactor was evaluated in this study, in terms of volatile fatty acid production and distribution, with respect to variations in hydraulic retention time (HRT). The continuous flow‐completely mixed anaerobic reactor, coupled with a conventional gravity settling tank and a continuous recycling system, was operated in a hydraulic retention time ranging between 24 and 12 h, and up to an organic loading rate of about 9.3 kg COD m^?3 d^?1, without pH control. The acid production gradually increased proportionally to the organic loading rate, with decrease in hydraulic retention time. The highest degree of acidification and the rate of acid production were 56% and 3.1 g dm^?3 d^?1 at 12 h of HRT. Variations in hydraulic retention time affected volatile fatty acid production and distribution substantially, for the range investigated. Acetic, propionic, butyric and valeric acids were commonly produced during acidogenesis of dairy wastewater. Copyright © 2004 Society of Chemical Industry     

A thermophilic/mesophilic dual digestion system for treating waste activated sludge

Five configurations for a dual digestion were examined. The units were based on three 5 dm³ completely stirred tank reactors (CSTR). A first‐stage thermophilic digester was used to provided the feed to each of the two second‐stage mesophilic (35 °C) digesters. The thermophilic digester was operated initially at 55 °C with a hydraulic retention time of 4 h and then, later, at 60 °C with a hydraulic retention time of 3 h. The mesophilic digesters were operated at hydraulic retention times of 9, 12 and 15 days. In terms of volatile solids’ (VS) reduction, the systems with a first stage at 60 °C gave the better performance. However, based on the specific methane yield (m³ CH₄ per kg VS removed), the configuration with a first stage operating at 55 °C and a secondary digester with a hydraulic retention time of 12 days was the most effective. © 1999 Society of Chemical Industry     

High‐rate clarification of municipal wastewaters: a brief appraisal

Pilot‐plant studies on the treatment of three wastewaters (biological aerated flooded filter (BAFF) backwash water, post‐primary settling tank effluent and BAFF effluent) have been conducted using a ballasted sedimentation process (ACTIFLO^®) coupled with ferric chloride and polyaluminium chloride (PACl) coagulants. The response of the process to shock loads of high solids concentrations was also investigated. Results showed over 80% turbidity, 70% suspended solids, and 50% COD removals to be achievable at rise rates in excess of 80 m h^?1 and dose rates of 10–20 mg dm^?3 Fe. Slightly improved performance was attained using PACL at half the weight concentration (but about the same molar concentration) as that of Fe. No pH adjustment was necessary and process performance was not significantly influenced by changes in hydraulic loading rate. Effluent quality was largely unaffected by shock organic loads provided coagulant dosing was commensurately increased. Non‐ideal flow through the plant was apparent from the measured residence time, which was ～25% less than that calculated for plug flow. Clarification data were comparable to those reported for other high‐rate clarification (HRC) processes, with somewhat improved performance in terms of hydraulic retention time and coagulant use. Copyright © 2004 Society of Chemical Industry     

Operation and Control of a Full-Scale Biogas Plant Treating Wastewater from the Cleaning of Car Tanks

A 1200-m³ full-scale biogas plant treating highly polluted wastewater (WW) from the cleaning of tank cars transporting food and fodder is working stably and efficiently despite the WW is constantly varying in strength and composition, is easily acidifying, and low in alkalinity. Na₂CO₃ has to be added to maintain the digester alkalinity and the digester pH at defined values due to softened water used for car tank cleaning. Monitoring volatile organic acid concentration and controlling the alkalinity in the digester are essential for maintaining the digestion process stability. Biogas production is adapted to the demand by a modulated feeding regime. The drastic modulation of feeding is possible due to the moderate organic loading of the plant along with the high average hydraulic retention time.     

The start‐up of anaerobic sequencing batch reactors at 20 °C and 25 °C for the treatment of slaughterhouse wastewater

The objective of this research was to evaluate the feasibility, the stability and the efficiency of a start‐up at 20 °C and 25 °C of anaerobic sequencing batch reactors (ASBRs) treating slaughterhouse wastewater. Influent chemical oxygen demand (COD) and suspended solids concentrations averaged 7500 and 1700 mg dm^?3, respectively. Reactor start‐up was completed in 168 and 136 days at 20 °C, and 25 °C, respectively. The start‐up process was stable at both temperatures, except for a short period at 20 °C, when effluent volatile fatty acid (VFA) concentrations increased from an average of 40 to 400 mg dm^?3. Effluent quality varied throughout start‐up, but in the last 25 days of the experiment, as the ASBRs were operated under organic loading rates of 2.25 ± 0.21 and 2.86 ± 0.24 kg m^?3 d^?1 at 20 °C and 25 °C, respectively, total COD was reduced by 90.3% ± 1.3%. Methanogenesis was not a limiting factor during start‐up. At 20 °C, the limiting factor was the acidification of the soluble organics and, to a smaller extent, the reduction of propionic, isobutyric and isovaleric acids into lower VFAs. At 25 °C, the limiting factor was the hydrolysis of particulate organics. To minimize biomass loss during the start‐up period, the organic loading rate should be increased only when 75 –80% of the COD fed has been transformed into methane within the design hydraulic retention time. © 2001 Society of Chemical Industry     

Straw bed priming enhances the methane yield and speeds up the start‐up of single‐stage,high‐solids anaerobic reactors treating plant biomass

A simple and potentially inexpensive implementation of a high‐solids reactor is a single‐stage, stratified bed reactor, in which the bed is made up of the plant biomass fed into the system. In the present study, the stratified bed was started up for a period of four weeks by either direct feeding of sugar beet leaves at four different feeding rates, or by introducing a straw bed primer which was batch digested without feeding. During weeks five to six both systems were fed with sugar beet leaves at such a rate that the total amount of beet leaves added at the end of week six was the same in each of the four corresponding pairs of straw and ‘no‐straw’ reactors. Straw bed priming enhanced the methane yield of the sugar beet leaves, with 0.33–0.37 m³ kg^?1 VS_added (volatile solids) accumulated at average solid retention times as short as 11–25 days, while the ‘no‐straw’ reactors had lower yields at longer average solid retention times. The levels and speciation of the organic acids suggested that both the rate and extent of the anaerobic digestion of the sugar beet leaves added in the straw reactors were improved. At the highest loading rate, the straw reactor failed, while the ‘no‐straw’ reactor did not. It is hypothesised that the microbial biomass was better established in the straw reactors than in the ‘no‐straw’ reactors. Copyright © 2006 Society of Chemical Industry     

Continuous methanogenesis of black liquor of pulp and paper mills in an anaerobic baffled reactor using an immobilized cell system

An anaerobic baffled reactor together with an immobilized cell system has been proposed for methanogenesis of the black liquor from pulp and paper mills in a continuous system. A maximum chemical oxygen demand reduction of 50%, and biogas generation of 10 L d^?1, having methane content of 66% (v/v) at an organic loading rate (OLR) of 7 kg m^?3 d^?1 with hydraulic retention time of 2 days, were recorded. OLR values higher than 7 kg m^?3 d^?1 were toxic to methanogenesis and destabilized the reactor system. Copyright © 2006 Society of Chemical Industry     

Mesophilic anaerobic digestion of corn thin stillage: a technical and energetic assessment of the corn‐to‐ethanol industry integrated with anaerobic digestion

BACKGROUND: The purpose of this study was to reduce the VS (volatile solid) and recover energy (methane) from thin stillage through mesophilic anaerobic digestion in corn–ethanol plants. The performance of a continuously stirred tank reactor (CSTR) with different hydraulic retention times (HRTs) was evaluated in this study. RESULTS: The results show no differences in volatile solid (VS) destruction (82–83%) in the reactor with HRTs ranging from 25 to 40 days. The maximum volumetric methane production rate of 1.41 L L^?1 day^?1 was produced at 25‐day HRT, whereas the maximum methane yield of approximately 0.63 L CH₄ g^?1 VS_fed (0.77 L g^?1 VS_removed) was achieved with HRTs between 30 and 40 days. Simulation results using a kinetic model indicate that the reactor needs to be operated for longer than 23 days in order to achieve 80% of maximum methane yield. The techno‐economic potential of a corn–ethanol facility to produce an estimated 57% energy recovery using mesophilic anaerobic digestion has long been overlooked. A corn–ethanol plant integrated with mesophilic anaerobic digestion increases the net energy balance ratio from 1.26 to 1.80. CONCLUSION: Mesophilic anaerobic digestion complements the corn–ethanol business so that the sustainable energy obtained from corn recovery is made more lucrative and renewable. Copyright © 2011 Society of Chemical Industry     

Reclamation of wastewater from a steel-making plant using an airlift submerged biofilm reactor

A bench-scale airlift submerged biofilm reactor was developed to test the possibility of nitrification of the final effluent discharged from a wastewater treatment process of a steel-making plant with an aim of reusing it as irrigation water. Despite the fluctuation of ammonia concentration in the wastewater (55–90 mg NH₃-N dm⁻³), the ammonia was completely converted to nitrate in the hydraulic retention time of 8 h. When decreasing the hydraulic retention time further down to 4 h, the nitrification efficiency decreased to 67·9%. However, the nitrification efficiency could be significantly enhanced by increasing the airflow rate due to an increase in both of the oxygen transfer rate and liquid circulation rate. At the aeration rate of 4 dm³ min⁻¹ and the hydraulic retention time of 4 h, the nitrification efficiency was as high as 92·6% and the nitrification rate was 34·6 mg NH₃-N dm⁻³ bed h⁻¹. © 1998 Society of Chemical Industry     

Mixing characteristics and performance of the anaerobic upflow blanket filter (UBF) reactor

Mixing characteristics (using a radioactive tracer) and reactor performance of the upflow blanket filter (UBF) reactor operated at different loading rates (up to 32 kg chemical oxygen demand (COD) m^?3 day^?1) were compared. The results indicated that mixing profiles of the reactor operated with effluent recirculation and without biomass were of the perfectly mixed type. Operation without recirculation resulted in about 18% dead space. The filter made of plastic rings and located in the top third of the reactor had no negative effect on reactor mixing. Operation at loading rates of up to 25 kg COD m^?3 day^?1 permitted a soluble COD removal rate of 95% with a methane production rate of 5.9 m³ m^?3 day^?1. At higher loading rates, the efficiency of COD reduction decreased with a decrease of the specific acetoclastic activity to 0.5 kg acetate removed per kg volatile suspended solids (VSS) day^?1. At all loading rates studied it was observed that the performance was not related to the mixing characteristics, which had remained of the perfectly mixed type with an occasional small dead space (below 10%). The good mixing characteristics of the UBF reactor coupled with the high biomass content and the effective action of the filter make this reactor one of the most promising designs for the treatment of soluble wastes.     

Anaerobic treatment of palm oil mill effluent by tank digesters

Palm oil mill effluent was treated anaerobically in 210 dm^?3 tank digesters in which inoculation with anaerobic seeding coupled with careful addition of lime could shorten the natural stabilisation process to about 30 days. The system was subsequently scaled up to 500 tonnes capacity. The characteristics of the anaerobically digested liquors were studied at 10 and 20 days hydraulic retention time (HRT) respectively, and close to 90% treatment efficiency could be achieved at 20 days HRT with complete microbial conversion of plant cell debris. However, the digested liquor still contained 0.325% (by wt) suspended solids. The kinetics of the anaerobic process show close resemblance to those treating other high strength organic wastes. Optimum gas production occurred only over a narrow pH range of 6.8–7.2 units. A methane content of 62–67% on gas production was noted about equivalent to 0.34 to 0.39 dm^?3 CH₄ g^?1 of BOD destroyed.     

Effect of retention time and organic loading rate on anaerobic acidification and biogasification of dairy manure

Vast amounts of animal manure produced from concentrated animal feeding operations have the potential to be converted into economic gain if the proper processing technology is employed. Anaerobic digestion (AD) is an effective way to convert organic wastes including animal manure into profitable by‐products as well as to reduce the pollution of water, air, and soil caused by these wastes. Two‐phase AD of manure offers several advantages over conventional one‐phase AD. Solids reduction through hydrolysis/acidification might be very significant for animal manure which contains high amounts of solids. However, to date, studies of two‐phase AD of animal manure have been limited to screened manure. Therefore, this study investigated the two‐phase AD of dairy manure with particular emphasis on the effects of retention time and organic loading rate (OLR) on anaerobic acidification and biogasification of unscreened dairy manure. The results indicated that pre‐acidification of dairy manure in daily‐fed continuously‐mixed reactors with no recycle led to significantly high reduction efficiencies of volatile solids and, thus, biogas production in the subsequent methanogenic reactor especially at OLRs of 4–10 g VS dm^?3 day^?1. However, the extent of the stimulation in the biogas production relative to corresponding feed samples was quite variable (between 6.9 and 64.7%) for different solids retention times and OLR combinations. A relatively lower performance was observed for the high OLRs (20–30 g VS dm^?3 day^?1) used which was attributed to the possible wash‐out of the acidifiers at the considerably low retention times (1.25–4 days) used. Copyright © 2004 Society of Chemical Industry     

Performance evaluation of the anaerobic fluidised bed system: I. Substrate utilisation and gas production

COD removal efficiencies in the range 75 to 98% were achieved in an anaerobic fluidised bed system designed for the recovery of methane from liquid wastes, when evaluated at COD loadings of between 5.8 to 108 kg m^?3 day^?1, hydraulic retention times of between 4.45 to 8 h, and feed COD concentrations of beween 480 to 9 000 mg dm^?3. More than 90% of feed COD could be removed up to COD loadings of about 40 kg m^?3 day^?1. Up to around 300 dm² of methane were produced per kg COD removed and this methane production rate was independent of the COD loadings applied in this investigation. Volatile acid concentration in the reactor increased sharply at a COD loading of about 40 kg m^?3 day^?1 and therefore, sufficient alkalinity should be provided to prevent pH from dropping to the undesirable level. The anaerobic fluidised bed system can be operated at a significantly higher liquid throughputs while maintaining its excellent efficiency.     

Effective treatment of night soil using anaerobic sequencing batch reactor (ASBR)

Laboratory experiments were conducted to investigate the performance of the anaerobic sequencing batch reactor (ASBR) process for night soil treatment. The reactors were evaluated at an equivalent hydraulic retention time (HRT) of 10 days with an equivalent loading rate of 2.6 kgVS m^-3day^-1 (3.1 kgCOD m^-3day,^-1) at 35 ‡C. Digestion of night soil was possible using the ASBR at an HRT of 10 days in spite of high concentration of ammonia nitrogen and settleable solids. Solids were accumulated rapidly in the ASBRs, and their concentrations were 2.3-2.4 times higher than that in a completely mixed control reactor. Increases in gas production were observed in the ASBRs compared with the control reactor. Average increases in equivalent daily gas production from the ASBRs were 205-220% compared with that from the control run. The ASBR with reaction period/thickening period (R/T) ratio of 1 showed a little higher gas production and organic removal efficiency than that with R/T ratio of 3. Volatile solids removal based on supernatant of the ASBRs was 12–14% higher than that of control reactor. Thus, the ASBR was a stable and effective process for the treatment of night soil having high concentration of settleable organics and ammonia nitrogen.     

Co-digestion of Municipal Sewage Sludges and Pre-hydrolysed Woody Agricultural Wastes

Material balance has been used to evaluate the COD behaviour and the time required for fed-batch digestion of mixtures of domestic sludges and pre-hydrolysed agricultural wastes. Pre-hydrolysis of the feed materials has been used to penetrate the strong lignocellulosic structure of these wastes as well as to increase the fraction of soluble organic substances in the mixture. The influence of the organic loading rate on the main process parameters (methane, carbon dioxide, total biogas productions and their respective conversion yields) has also been investigated. The organic load has been varied from 0·8 up to 6·1 g_COD dm⁻³ day⁻¹, corresponding to a range of volatile solids load of 0·6–4·5 g_VS dm⁻³ day⁻¹ for the material under consideration. These values are slightly higher than those usually employed in conventional digester for domestic sewage sludges. However, methane production reached a maximum rate of only 5·6 mmol dm⁻³ day⁻¹ at an organic loading rate of 4·6 g_COD dm⁻³ day⁻¹, while both CH₄ content and production of biogas rapidly fell over 2·2 g_COD dm⁻³ day⁻¹. On the whole, these results suggest that removal of lignin is necessary in order to carry out the continuous anaerobic digestion of pre-hydrolysed agricultural wastes rich in woody materials. © 1997 SCI.     

Integration of continuous biological and chemical (ozone) treatment of domestic wastewater: 1. Biodegradation and post‐ozonation

The continuous treatment of domestic wastewater by an activated sludge process and by an integrated biological–chemical (ozone) oxidation process were studied in this work. Chemical oxygen demand (COD), biochemical oxygen demand (BOD), absorbance at 254 nm (UV₂₅₄) and nitrogenous compound content were the parameters followed in order to evaluate the performance of the two processes. Experimental data showed that both UV₂₅₄ and COD reductions are improved in the combined biological–chemical oxidation procedure. Thus, reductions of 59.1% and 37.2% corresponding to COD and UV₂₅₄, respectively were observed after the biological process (hydraulic retention time = 5 h; mixed liquor volatile suspended solids concentration = 3142 g m⁻³) compared with 71.0% and 78.4% obtained when a post‐ozonation step ( D _O3 = 41.7 g m⁻³) was included. During conventional activated sludge treatment, appropriate nitrification levels are only achieved with high hydraulic retention time and/or biomass concentration. Ozonation after the secondary treatment, however, allows improved nitrogen content reduction with total nitrite elimination. Post‐ozonation also leads to a higher biodegradability of the treated wastewater. Thus, the ultimate BOD/COD ratio goes from 0.16 after biological oxidation to 0.34 after post‐ozonation with 41.7 g O₃ m⁻³. © 1999 Society of Chemical Industry     

Optimizing design and cost of seawater reverse osmosis systems

The influence of design parameters, such as raw water temperature and total dissolved solids content (TDS), design pressure, and recovery, on total unit cost for the production of desalinated water from the Arabian Gulf in Saudi Arabia by means of reverse osmosis is presented for plant capacities ranging from 75,000 m³/d to 700,000 m³/d. It is concluded that total unit water cost is significantly influenced by raw water TDS, design pressure and design recovery. It is recommended to investigate the feasibility of operation at increased pressure. Finally, total water costs for reverse osmosis desalination are presented also as a function of salinity, with TDS levels ranging from 3000 mg/l to 55,000 mg/l.     

循环生物曝气滤池和过滤组合工艺处理炼油轻度污染废水

采用新型的循环生物曝气滤池（CBAF）和过滤组合工艺对炼油轻度污染废水进行净化回用工业试验。研究了填料粒径和高度、水力停留时间和溶解氧浓度对CBAF工艺处理效果的影响。结果表明CBAF工艺具有碳化作用、硝化作用和过滤作用。CBAF工艺净化该废水适宜的操作条件为：水力停留时间100 min,溶解氧浓度3 mg·L^-1左右,反冲洗周期2～3 d。炼油轻度污染废水经该组合工艺处理后,COD、石油类污染物、NH₃-N和SS平均去除率分别为62.6%、71.7%、92.6%和97.0%,出水COD、石油类污染物、NH₃-N和SS平均质量浓度分别为14.4 mg·L^-1、0.75 mg·L^-1、0.49 mg·L^-1 和2.4 mg·L^-1,经处理后出水水质达到工业回用水要求。     

城市污水处理厂污泥中温两相厌氧消化的研究

采用中温两相厌氧消化处理西安市污水处理厂污泥,结果表明,污泥可在较短的停留时间(296 h,产酸相30 h,产甲烷相266 h),较高的有机负荷(约2.65 kg VS/m3.d)下稳定运行,系统VS去除率38.87%,pH≈7.5,产气速率6.33 L/L.d,单位VS产气率3.00 L/g。与单相厌氧消化处理结果对比,两相厌氧消化系统的停留时间可减少一半,而污泥有机负荷提高2倍,且有机物的去除率大,处理效果好,稳定性高,抗冲击负荷能力强,产气性良好。     

制糖废水连续流厌氧发酵制氢系统的运行特性

采用连续流搅拌槽式反应器(CSTR)作为反应装置,探讨了制糖废水厌氧发酵法生物制氢的可行性与运行特征。研究表明,在污泥接种量(以挥发性悬浮固体计)为17.74 g/L,温度为(35±1)℃,水力停留时间(HRT)为6 h,通过调节有机负荷,在12 d左右就可以快速实现生物制氢反应器中微生物的主要代谢类型为乙醇型发酵;而且此时的CSTR产氢发酵系统对负荷冲击表现出了良好的调节能力,在有机负荷(以化学需氧量COD计)从8 kg/(m3.d)提高到24 kg/(m3.d)时,反应系统可在9 d内重新达到稳定运行状态,其COD去除率和产气量由8%和3 L/d提高到20%和12 L/d,发酵气中氢气体积分数为67%。