Integration of sulphate reduction, autotrophic denitrification and nitrification to achieve low-cost excess sludge minimisation for Hong Kong sewage.

An integrated anaerobic-aerobic treatment system of sulphate-laden wastewater was proposed here to achieve low sludge production, low energy consumption and effective sulphide control. Before integrating the whole system, the feasibility of autotrophic denitrification utilising dissolved sulphide produced during anaerobic treatment of sulphate rich wastewater was studied here. An upflow anaerobic sludge blanket reactor was operated to treat sulphate-rich synthetic wastewater (TOC=100 mg/L and sulphate=500 mg/L) and its effluent with dissolved sulphide and external nitrate solution were fed into an anoxic biofilter. The anaerobic reactor was able to remove 77-85% of TOC at HRT of 3 h and produce 70-90 mg S/L sulphide in dissolved form for the subsequent denitrification. The performance of anoxic reactor was stable, and the anoxic reactor could remove 30 mg N/L nitrate at HRT of 2 h through autotrophic denitrification. Furthermore, sulphur balance for the anoxic filter showed that more than 90% of the removed sulphide was actually oxidised into sulphate, thereby there was no accumulation of sulphur particles in the filter bed. The net sludge productions were approximately 0.15 to 0.18 g VSS/g COD in the anaerobic reactor and 0.22 to 0.31 g VSS/g NO3- -N in the anoxic reactor. The findings in this study will be helpful in developing the integrated treatment system to achieve low-cost excess sludge minimisation.     

Anaerobic digestion of blackwater from vacuum toilets and kitchen refuse in a continuous stirred tank reactor (CSTR).

The objective of this research was mesophilic anaerobic digestion of blackwater from vacuum toilets (BW) and kitchen refuse (KR) in a CSTR within an ecological sanitation system. A detailed investigation of the BW characteristics was carried out. Research on anaerobic digestion was performed with CSTR of 101 volume at HRT of 10, 15 and 20 days. The digestion of BW at 20 days HRT showed stable performance without inhibition effects, in spite of relatively high ammonium concentrations. The removal of total and particulate COD was 61% and 53%, respectively, and the methane yield 10/CH4/cap/day. The addition of kitchen refuse (KR) improved the performance of the CSTR in terms of COD removal efficiency and methane yield. At 20 days HRT the removal of total and particulate COD increased up to 71% and 67%, respectively, and the methane yield to 27/CH4/cap/day. The results at 15 days HRT showed similar performance. At HRT of 10 days, the anaerobic treatment was limited but reached steady state conditions at higher VFA concentrations in the effluent, with a decrease of COD removal of 30 to 33% and of methane yields of 19 to 21%.     

Methanogenic and sulphate reducing bacterial population levels in a full-scale anaerobic reactor treating pulp and paper industry wastewater using fluorescence in situ hybridisation.

In this study, specific methanogenic activity (SMA) test and fluorescence in situ hybridisation (FISH) were respectively used to determine acetoclastic methanogenic capacity, and composition and number of methanogenic and sulphate reducing bacterial (SRB) populations within a full scale anaerobic contact reactor treating a pulp and paper industry effluent. The sludge samples were collected from three different heights along the anaerobic reactor having a difficulty of completely stirring. Performance of the anaerobic reactor in terms of COD removal efficiency varied between 47 and 55% at organic loading rates in a range of 1.6-1.8 kg COD m(-3) d(-1) and methane yield varied between 0.18 and 0.20 m3CH4kg CODrem(-1). The anaerobic reactor was not operated for 2 weeks during the monitoring period. According to SMA test results, potential methane production rate was 276 mLCH4 gVSS(-1) d(-1) before the off period of the reactor, however it decreased to 159 mL CH4 gVSS(-1) d(-1) after this period. SMA test and FISH results along the reactor height showed that the acetoclastic methanogenic activity of the sludge samples, the relative abundance of acetoclastic methanogens, hydrogenotrophic methanogens and acetate oxidising SRB decreased as the reactor height increased, however the relative abundance of non-acetate oxidising SRB increased.     

Evaluation of the reject waters from co-digestion of solid wastes from agro-industries in a municipal WWTP.

Overcapacities of anaerobic digesters at municipal WWTPs are frequently used for the treatment of organic wastes in order to increase the biogas production. However, "co-digestion" of organic wastes leads to additional nitrogen loading and to additional loads of non-biodegradable COD. The effects of (co-) digestion of organic wastes from agro-industries (slaughterhouses, dairies and leather industry) on the wastewater cycle have been evaluated in full-scale investigations at Leoben WWTP with a capacity of 90,000 pe where the methane production was increased from 700 to more than 1700 Nm3 CH4 per day. For this evaluation, mass balances for COD and nitrogen have been applied to estimate the fluxes of these substances. Application of this method is described in detail. As the additional loadings, it was found that related to methane production less nitrogen is released from the organic wastes than from the waste sludge. While the ammonia nitrogen load in the effluent from sludge digestion was about 100 g NH4-N per Nm3 of CH4 produced, in the effluent from the digestion of organic wastes only 70 g NH4-N/Nm3 CH4 were found. The decrease in the COD removal efficiency after digestion of the organic wastes started was not regarded as significant enough to be seen as a consequence of the treatment of external substrate.     

Methane and hydrogen sulfide emissions in UASB reactors treating domestic wastewater

The release of CH(4) and H(2)S in UASB reactors was evaluated with the aim to quantify the emissions from the liquid surfaces (three-phase separator and settler compartment) and also from the reactor's discharge hydraulic structures. The studies were carried out in two pilot- (360 L) and one demo-scale (14 m(3)) UASB reactors treating domestic wastewater. As expected, the release rates were much higher across the gas/liquid interfaces of the three-phase separators (5.4-9.7 kg CH(4) m(-2) d(-1) and 23.0-35.8 g S m(-2) d(-1)) as compared with the quiescent settler surfaces (11.0-17.8 g CH(4) m(-2) d(-1) and 0.21 to 0.37 g S m(-2) d(-1)). The decrease of dissolved methane and dissolved hydrogen sulfide was very large in the discharging hydraulic structures very close to the reactor (>60 and >80%, respectively), largely due to the loss to the atmosphere, indicating that the concentration of these compounds will probably fall to values close to zero in the near downstream structures. The emission factors due to the release of dissolved methane in the discharge structure amounted to around 0.040 g CH(4) g COD(infl)(-1) and 0.060 g CH(4) g COD(rem)(-1), representing around 60% of the methane collected in the three-phase separator.     

Pretreatment of silk-dyeing industrial wastewater by UASB reactor.

The objective of this study was to investigate the performance of the upflow anaerobic sludge blanket (UASB) reactor as the pretreatment system for silk-dyeing wastewater. Two laboratory-scale UASB reactors, with working volume of 15.59 I, were used during May 1998 to June 1999. The actual wastewater was diluted to reduce ammonium ion toxicity on anaerobic bacteria. The experiments were conducted at the organic loading rates (OLRs) of 0.52, 1.01, 1.04, 1.54 and 2.56 kgCOD/(m3 x d), treating only wastewater generated from the acid-dye process of mixed-species raw silk. It took approximately 4 1/2 months to reach the steady-state conditions. It was found that the COD removal was in the ranges of 74.1-85.3%, except at OLR 2.56 kgCOD/(m3 x d) where efficiency significantly dropped to 55.2%. The apparent color removal was in the similar trend as COD. During the study periods, wastewater input had various color shades while the effluent generally looked pale yellowish. The methane generation rates ranged from 0.18-0.31 m3/kg COD removed, with methane composition 81.0-88.1% in biogas. The average granule size in the sludge bed had slowly increased to 0.73 mm in the last experiment. It can be concluded that the UASB reactor is suitable as a pretreatment system for silk-dyeing wastewater. An OLR of 1 kgCOD/(m3 x d) and an influent concentration diluted to 2,600 mgCOD/l are suggested while COD and apparent color removal efficiency of 80% and 70%, respectively, can be expected.     

Treatment of low and medium strength sewage in a lab-scale gradual concentric chambers (GCC) reactor.

One of the major challenges of anaerobic technology is its applicability for low strength wastewaters, such as sewage. The lab-scale design and performance of a novel Gradual Concentric Chambers (GCC) reactor treating low (165+/-24 mg COD/L) and medium strength (550 mg COD/L) domestic wastewaters were studied. Experimental data were collected to evaluate the influence of chemical oxygen demand (COD) concentrations in the influent and the hydraulic retention time (HRT) on the performance of the GCC reactor. Two reactors (R1 and R2), integrating anaerobic and aerobic processes, were studied at ambient (26 degrees C) and mesophilic (35 degrees C) temperature, respectively. The highest COD removal efficiency (94%) was obtained when treating medium strength wastewater at an organic loading rate (OLR) of 1.9 g COD/L.d (HRT = 4 h). The COD levels in the final effluent were around 36 mg/L. For the low strength domestic wastewater, a highest removal efficiency of 85% was observed, producing a final effluent with 22 mg COD/L. Changes in the nutrient concentration levels were followed for both reactors.     

Start-up and steady-state results of a full-scale UASB reactor treating malting wastewater.

An Imhoff tank was reconstructed into a 250 m3 UASB reactor in order to treat a malting plant wastewater. The UASB was inoculated with sludge from an anaerobic lagoon used for slaughterhouse wastewater treatment. After two months of operation the reactor achieved full load with an HRT of 17 h, a COD removal higher than 80% and a biogas production of 300 m3/day (77% average methane content), with an organic loading rate of 3.6 kgCOD/m3.d (0.24 kgCOD/kgVSS.d). A yield coefficient of 0.09 gVSS/gCODrem was found from a mass balance. The fat present in the inoculated sludge (48 mg/gSSV) did not affect the start up performance. Sludge from the inoculum with high content of fat (270 mg/gSSV), was separated by flotation in the first week of operation. The COD removal efficiency was scarcely influenced by the reactor operation temperature (17-25 degrees C).     

Comparison of methane production by co-digesting fruit and vegetable waste with first stage and second stage anaerobic digester sludge from a two stage digester

Fruit and vegetable waste (FVW) was co-digested with first stage (FSS) and second stage anaerobic digester sludge (SSS) separately, over the course of 10 days, in batch reactors. Addition of FVW significantly increased the methane production in both sludges. After 10 days of digestion FSS + FVW produced 514 ± 57 L CH(4) kg VS(-1)(added) compared with 392 ± 16 L CH(4) for the SSS + FVW. The increased methane yield was most likely due to the higher inoculum substrate ratio of the FSS. The final VS and COD contents of the sewer sludge and FVW mixtures were not significantly different from the control values suggesting that all of the FVW added was degraded within 10 days. It is recommended that FVW be added to the first stage of the anaerobic digester in order to maximize methane generation.     

Quantification of dissolved methane in UASB reactors treating domestic wastewater under different operating conditions

This paper aimed at measuring the concentration of methane dissolved in effluents from different UASB reactors (pilot-, demo- and full-scale) treating domestic wastewater, in order to calculate the degree of saturation of such greenhouse gas and evaluate the losses of energetic potential in such systems. The results showed that methane saturation degrees, calculated according to Henry's law, varied from ～1.4 to 1.7 in the different reactors, indicating that methane was oversaturated in the liquid phase. The overall results indicated that the losses of dissolved methane in the anaerobic effluents were considerably high, varying from 36 to 41% of total methane generated in the reactor. These results show that there is considerable uncontrolled loss of methane in anaerobic wastewater treatment plants, implying the need of research on technologies aimed at recovering such energetic greenhouse gas.     

变温下土地滤渗系统氧化还原环境特征的研究

以毛管逆滤渗系统处理生活污水的实验为例,研究变温下污水土地滤渗系统的氧化还原环境。结果表明:氧化还原电位(ORP)随着系统的进水—落干交替而高低起伏,使系统依次出现厌氧、好氧环境,并且距污染源越近,ORP起伏变化越明显;ORP与温度、进水浓度及污水过滤速度均呈负相关关系;ORP与COD处理效果的关系则表现为ORP值高,出水COD浓度普遍较低,COD去除率高。     

Anaerobic co-digestion of winery wastewater.

The operational performance of anaerobic batch reactors treating winery wastewater (WW) combined with waste activated sludge (WAS) in different proportions was investigated under mesophilic conditions. In these experiments it was shown that for anaerobic digestion of WW alone, methane production rate was lower than the rates achieved when WW and WAS were treated together. When WW was mixed with WAS at a concentration of 50% WW resulted in the highest methane production rates. A simplified anaerobic model was used to determine the main kinetic parameters; maximum COD reduction rate (q(DA)) and maximum methane generation rate (k(max)). The maximum values of q(DA) and k(max) were 16.50 kgCOD COD(-1) d(-1) and 14.34 kgCOD kgCOD(-1) d(-1), respectively.     

Anaerobic pond treatment of wastewater containing sulphate.

Anaerobic ponds are usually used for treatment of industrial and agricultural wastes which contain high organic matter and sulphate. Competition for substrate between sulphate reducing bacteria and methane producing archaea, and the inhibitory effects of sulphide produced from microbial sulphate reduction reported in the literature varied considerably. In this research, a laboratory scale column-in-series anaerobic pond reactor, consisting of five cylindrical columns of acrylic tubes, was operated to evaluate the effect of COD and sulphate ratio on pond performance treating wastewater containing high organic matter and sulphate from a tapioca starch industry. The result depicted that no adverse effect of COD:SO4 ratios between 5 and 20 on overall COD removal performance of anaerobic pond operated with organic loading rate (OLR) of 150 to 600 g COD/m3d. Sulphate reducing bacteria could out-compete methane producing archaea for the same substrate at COD:SO4 ratio equal to or lower than 5 and OLR greater than 300 g COD/m3d. Sulphide inhibition was not observed on overall performance of pond up to an influent sulphate concentration of 650 mg/L.     

Hydroponic system for the treatment of anaerobic liquid

The effluent from anaerobic digestion process has high concentrations of nutrients, particularly nitrogen, essential for plant growth but is not suitable for direct disposal or application due to high chemical oxygen demand (COD), low dissolved oxygen (DO), odour issues and is potentially phytotoxic. This research explored the optimum conditions of anaerobic effluent for application and dilutions of the effluent required to obtain better plant growth. A small-scale hydroponic system was constructed in a glasshouse to test different concentrations of anaerobic effluent against a commercial hydroponic medium as the control for the growth of silverbeet. It was found that the survival of silverbeet was negatively affected at 50% concentration due to low DO and NH(4) toxicity. The concentration of 20% anaerobic liquid was found to be the most efficient with highest foliage yield and plant growth. The hydroponic system with 20% concentrated effluent had better utilisation of nutrients for plant growth and a COD reduction of 95% was achieved during the 50-day growth period. This preliminary evaluation revealed that the growth and development of silverbeet was significantly lower in anaerobic effluent compared with a commercial hydroponic plant growth solution. The nutrient quality of anaerobic effluent could be highly variable with the process and the waste material used and dilution may depend on the nutrient content of the effluent. It is recommended that, a pre-treatment of the effluent to increase DO and reduce ammonium content is required before plant application, and simple dilution by itself is not suitable for optimum plant growth in a hydroponic system.     

Winery and distillery wastewater treatment by anaerobic digestion.

Anaerobic digestion is widely used for wastewater treatment, especially in the food industries. Generally after the anaerobic treatment there is an aerobic post-treatment in order to return the treated water to nature. Several technologies are applied for winery wastewater treatment. They are using free cells or flocs (anaerobic contact digesters, anaerobic sequencing batch reactors and anaerobic lagoons), anaerobic granules (Upflow Anaerobic Sludge Blanket--UASB), or biofilms on fixed support (anaerobic filter) or on mobile support as with the fluidised bed. Some technologies include two strategies, e.g. a sludge bed with anaerobic filter as in the hybrid digester. With winery wastewaters (as for vinasses from distilleries) the removal yield for anaerobic digestion is very high, up to 90-95% COD removal. The organic loads are between 5 and 15 kgCOD/m3 of digester/day. The biogas production is between 400 and 600 L per kg COD removed with 60 to 70% methane content. For anaerobic and aerobic post-treatment of vinasses in the Cognac region, REVICO company has 99.7% COD removal and the cost is 0.52 Euro/m3 of vinasses.     

Anaerobic digestion potential for ecological and decentralised sanitation in urban areas.

The potential of anaerobic digestion in ecological and decentralised sanitation has been investigated in this research. Different anaerobic digestion systems were proposed for the treatment of sewage, grey water, black water and faeces. Moreover, mathematical models based on anaerobic digestion model no.1 (ADM1) were developed for determination of a suitable design for each system. For stable performance of an upflow anaerobic sludge blanket (UASB) reactor treating sewage, the model results indicated that optimisation of wastewater conversion to biogas (not COD removal) should be selected for determination of the hydraulic retention time (HRT) of the reactor. For the treatment of sewage or black water in a UASB septic-tank, the model results showed that the sludge removal period was the main parameter for determination of the HRT. At such HRT, both COD removal and wastewater conversion are also optimised. The model results demonstrated that for treatment of faeces in an accumulation (AC) system at temperature > or = 25 degrees C, the filling period of the system should be higher than 60 days. For maximisation of the net biogas production (i.e. reduction of biogas losses as dissolved in the effluent), the separation between grey water, urine and faeces and reduction of water consumption for faeces flushing are required. Furthermore, the faeces and kitchen organic wastes and grey water are digested in, respectively, an AC system and UASB reactor, while the urine is stored.     

Strategy for olive mill wastewater treatment and reuse with a sewage plant in an arid region.

This study was conducted to evaluate the treatability of OMW (olive mill wastewater) with sewage and sewage sludge, which could supplement nutrients and microbes required for OMW treatment and reduce its possible toxicity. The amount of OMW added to an aeration tank was based on the loading difference between the designed and actual COD loads, while the amount added to anaerobic digestion for energy recovery was determined by CH4 production. The COD removal efficiencies were 70-85% for both systems. Compost of OMW with dried sewage sludge also showed a similar temperature profile without OMW addition. This strongly suggested that OMW can be treated at a sewage plant without pretreatment and the treated effluent can be reused in irrigation for an arid region.     

Anaerobic digestion elutriated phased treatment of piggery waste.

The performance of a novel high-rate anaerobic process, the anaerobic digestion elutriated phased treatment (ADEPT) process, for treating a slurry-type piggery waste (55 g COD/L and 37 g TS/L) was investigated. The ADEPT process consists of an acid elutriation slurry reactor for hydrolysis and acidification, followed by an upflow anaerobic sludge bed reactor for methanification. This process provides stable and high system performance with short HRT (7.4 d) and better effluent quality (2 g SCOD/L and 0.68 g VSS/L) due to the alkaline pH condition for hydrolysis/acidification phase, high refractory solids removal and ammonia toxicity reduction. The optimum pH and HRT for hydrolysis/acidogenesis of the piggery waste were 9 and 5 days at both 35 degrees C and 55 degrees C conditions. The hydrolysis and acidification rate in the mesophilic reactor were 0.05 d(-1) and 0.11 d(-1), meaning that hydrolysis was a limiting step. SCOD production by the hydrolysis was about 0.26 g SCOD/g VS(fed) (3.6 g SCOD/g VS reduction). Methane production and content in the system were 0.3 L CH4/g VS(fed) (0.67 L CH4/g VS destroyed) and 80%, respectively, corresponding to 0.23 L CH4/g COD removal (@STP).     

Biogas production from anaerobic waste stabilisation ponds treating dairy and piggery wastewater in New Zealand.

New Zealand has over 1000 anaerobic wastewater stabilisation ponds used for the treatment of wastewater from farms and industry. Traditional anaerobic ponds were not designed to optimise anaerobic digestion of wastewater biomass to produce biogas and these uncovered ponds allowed biogas to escape to the atmosphere. This release of biogas not only causes odour problems, but contributes to GHG (greenhouse gas) emissions and is wasteful of energy that could be captured and used. Biogas production from anaerobic stabilisation ponds treating piggery and dairy wastewater was measured using floating 25 m2 HDPE covers on the pond surface. Biogas composition was analysed monthly and gas production was continually monitored. Mean areal biogas (methane) production rates from piggery and dairy anaerobic ponds were 0.78 (0.53) m3/m2/d and 0.03 (0.023) m3/m2/d respectively. Average CH4 content of the piggery and dairy farm biogas were 72.0% and 80.3% respectively. Conversion of the average volume of methane gas that could be captured from the piggery and dairy farm ponds (393.4 m3/d and 40.7 m3/d) to electricity would reduce CO2 equivalent GHG emissions by 5.6 tonnes/d and 0.6 tonnes/d and generate 1,180 kWh/d and 122 kWh/d. These results suggest that anaerobic ponds in New Zealand release considerable amounts of GHG and that there is great potential for energy recovery.     

ADM1 performance using SS-OFMSW with non-acclimated inoculums

This paper assesses the anaerobic digestion (AD) of the source-sorted organic fraction of municipal solid waste (SS-OFMSW). For this purpose, an experimental programme was implemented involving the operation and monitoring of two bench-scale anaerobic digesters, continuously fed with SS-OFMSW. The mathematical model (ADM1) was then applied to simulate the process of AD of SS-OFMSW. While start-up of the digesters was relatively slow, re-inoculation with cattle manure with effluent dilution reduced the acclimation period and achieved better stability, accommodating a feeding rate at an OLR = 2.39 kg TVS m(-3) day(-1). The high recorded methane gas production rate, reaching (0.1-2.5 m(3) CH(4)/m(3) reactor day), confirms the excellent biodegradability of the type of waste used (SS-OFMSW) and its suitability for AD. Satisfactory simulations of soluble chemical oxygen demand (COD), pH, and methane composition of biogas were obtained, whereas volatile fatty acid (VFA) concentrations in both reactors were over-predicted albeit capturing its general trend.