The role of the headspace in hydrogen sulfide removal during microaerobic digestion of sludge

The role of the headspace (HS) in the microaerobic removal of hydrogen sulfide from biogas produced during sludge digestion was studied. Research was carried out in a pilot reactor with a total volume of 265 L, under mesophilic conditions. Biogas was successfully desulfurized (99%) by introducing pure oxygen (0.46 NL/L(fed)) into the recirculation stream when the HS volume was both 50.0 and 9.5 L. The removal efficacy dropped sharply to ≈15% when the HS was reduced to 1.5 L. The system responded quickly to the operational changes imposed: micro-oxygenation stops and variations in supply, as well as HS volume reductions and increases. As the final result, the microaerobic process required a minimum surface into the gas space to occur, which along with the elemental sulfur deposition in this area indicated that the oxidation took place there. Additionally, the pattern of sulfur accumulation suggested that the removal occurred preferentially on certain materials, and pointed to a significant biological contribution.     

Comparison of microbial activity in anaerobic and microaerobic digesters

Microaerobic alternative of anaerobic digestion offers many advantages especially when sulfide concentration in the digester is high. For better understanding of the microaerobic technology more detailed characterization of biomass activity is needed. Two equal digesters were operated under the same condition except of microaeration in one of them. During long term operation of anaerobic and microaerobic digesters the sludge quality and the biomass activity was monitored. The activity of sulfide oxidizing bacteria of microaerobic biomass was significantly higher in comparison with anaerobic biomass. The activity of sulfate reducing bacteria was comparable. The activity of methanogenic bacteria activity depended on sulfide concentration more than on microaeration. The extent of foaming problems was lower in the microaerobic than in the anaerobic digester.     

Determination of the optimal rate for the microaerobic treatment of several H2S concentrations in biogas from sludge digesters

The treatment of H2S in the biogas produced during anaerobic digestion has to be carried out to ensure the efficient long-lasting use of its energetic potential. The microaerobic removal of H2S was studied to determine the treatment capacity at low and high H2S concentrations in the biogas (0.33 and 3.38% v/v) and to determine the optimal O2 rate that achieved a concentration of H2S of 150 mg/Nm3 or lower. Research was performed in pilot-plant scale digesters of sewage sludge, with 200 L of working volume, in mesophilic conditions with a hydraulic retention time of 20 d. O2 was supplied at different rates to the headspace of the digester to create the microaerobic conditions. The treatment successfully removed H2S from the biogas with efficacies of 97% for the low concentration and 99% for the highest, in both cases achieving a concentration below 150 mg/Nm3. An optimal O2 rate of 6.4 NLO2/Nm3 of biogas when treating the biogas was found with 0.33% (v/v) of H2S and 118 NLO2/ Nm3 of biogas for the 3.38% (v/v) concentration. This relation may be employed to control the H2S content in the biogas while optimising the O2 supply.     

Two-stage thermophilic-mesophilic anaerobic digestion of waste activated sludge from a biological nutrient removal plant.

A two-stage thermophilic-mesophilic anaerobic digestion pilot-plant was operated solely on waste activated sludge (WAS) from a biological nutrient removal (BNR) plant. The first-stage thermophilic reactor (HRT 2 days) was operated at 47, 54 and 60 degrees C. The second-stage mesophilic digester (HRT 15 days) was held at a constant temperature of 36-37 degrees C. For comparison with a single-stage mesophilic process, the mesophilic digester was also operated separately with an HRT of 17 days and temperature of 36-37 degrees C. The results showed a truly thermophilic stage (60 degrees C) was essential to achieve good WAS degradation. The lower thermophilic temperatures examined did not offer advantages over single-stage mesophilic treatment in terms of COD and VS removal. At a thermophilic temperature of 60 degrees C, the plant achieved 35% VS reduction, representing a 46% increase compared to the single-stage mesophilic digester. This is a significant level of degradation which could make such a process viable in situations where there is no primary sludge generated. The fate of the biologically stored phosphorus in this BNR sludge was also investigated. Over 80% of the incoming phosphorus remained bound up with the solids and was not released into solution during the WAS digestion. Therefore only a small fraction of phosphorus would be recycled to the main treatment plant with the dewatering stream.     

Anammox brings WWTP closer to energy autarky due to increased biogas production and reduced aeration energy for N-removal.

Fifty years ago when only BOD was removed at municipal WWTPs primary clarifiers were designed with 2-3 hours hydraulic retention time (HRT). This changed with the introduction of nitrogen removal in activated sludge treatment that needed more BOD for denitrification. The HRT of primary clarification was reduced to less than one hour for dry weather flow with the consequence that secondary sludge had to be separately thickened and biogas production was reduced. Only recently the ammonia rich digester liquid (15-20% of the inlet ammonia load) could be treated with the very economic autotrophic nitritation/anammox process requiring half of the aeration energy and no organic carbon source compared to nitrification and heterotrophic denitrification. With the introduction of this new innovative digester liquid treatment the situation reverts, allowing us to increase HRT of the primary clarifier to improve biogas production and reduce aeration energy for BOD removal and nitrification at similar overall N-removal.     

Study of the biological N removal over nitrite in a physico-chemical-biological treatment of digested pig manure in a SBR.

SBR technology is used to treat the supernatant from mesophilic anaerobic digestion of piggery wastewater. The novelty of the treatment consists in the use of a final coagulation/flocculation step inside the SBR cycle to reach the legal COD effluent standard. The pH changes introduced by the use of FeCl(3) do not affect the nitrifying activity. The SBR treatment includes a strategy to the control of oxygen supply and ammonia concentrations inside the digester to favor the biological nitrogen removal over nitrite, which makes the process more economical. The influence of several of these parameters on the AOB biomass activity is studied in this paper.     

Anaerobic digestion of slaughterhouse waste with UF-membrane separation and recycling of permeate after free ammonia stripping.

Anaerobic digestion can adapt to free ammonia to a certain extent. During the anaerobic digestion of slaughterhouse waste, however, an ammonia concentration of up to 15 g Nl(-1) can be reached in the sludge liquid and this will even inhibit adapted sludge. To lower this concentration, a fraction of the digester liquid must therefore be continuously separated from the digested sludge and the free ammonia stripped before the liquid is recycled to the digester. A mesophilic laboratory digester was successfully operated with an ammonium concentration of 4-5g l(-1) and a pH of 8.0-8.4. After free ammonia stripping, the excess liquid was treated in a laboratory SBR for nitrogen and phosphorus removal before being added to the receiving water. The effluent had no toxic effect on daphnia and algae.     

Performance of high-rate sludge digesters fed with sonicated sludge.

A major limitation of anaerobic sludge digestion is the long hydraulic retention time (HRT) required for satisfactory stabilization which results in large digester size. This study explored a possibility of operating digesters at shortened HRTs by sonication pretreatment of secondary sludges. Four identical digesters designated D1, D2, D3 and D4 were fed with untreated and sludge sonicated at densities of 0.18 W/ml, 0.33 W/ml and 0.52 W/ml, respectively. All digesters were operated at three HRTs of 8-day, 4-day and 2-day. Comparing with the control digester (D1), total solids removal efficiencies improved by 12-19%, 17-36% and 20-39% in digesters D2, D3 and D4, respectively. The volatile solids removal was also increased by 11-21%, 17-33% and 19-36% in the respective digesters. The improved solids degradation corresponded with increase in biogas production by 1.4-2.5, 1.9-3.0 and 1.6-3.1 times, respectively. Increase in methane composition by 2-17% was also noted in all digesters fed with sonicated sludge. An analysis indicated that sonication pretreatment could enhance degradation of carbon, nitrogen and sulfur substances in the digestion. The study suggested that sonication of sludge is a possible pretreatment to shorten the digester operating HRT with improvement in solids degradation, biogas production and methane content. It can be deduced that to maintain a consistent solids loading at a desire performance, sludge digester with smaller size can be designed.     

Methanotrophic biodegradation of cis-1,2-dichloroethylene in a continuously fed fixed-film bioreactor.

Co-metabolic biodegradation of cis-dichloroethylene (cis-DCE) was investigated in a bench-scale fixed-film bioreactor inoculated with a mixed culture of methane oxidising bacteria. The aim of this work was to identify factors that affect the cis-DCE biodegradation. It was observed that the presence of methane was necessary to enhance the biodegradation of cis-DCE, but an excess of methane inhibited the cis-DCE removal. cis-DCE did not inhibit the methane biodegradation at concentrations up to 300 microg/L. Maximum cis-DCE removal was observed with a methane bulk concentration ranging from 0.2 to 0.7 mg/L. It was found that the activity of the biofilm was located in the upper 100 microm of the biofilm. On the basis of this study it is concluded that careful control of the oxygen and methane concentrations as well as of the biofilm thickness is necessary in order to optimise the biodegradation of cis-DCE in fixed film bioreactors.     

Lab scale study on electrocoagulation defluoridation process optimization along with aluminium leaching in the process and comparison with full scale plant operation

An excess or lack of fluoride in drinking water is harmful to human health. Desirable and permissible standards of fluoride in drinking water are 1.0 and 1.5 mg/L, respectively, as per Indian drinking water quality standards i.e., BIS 10500, 1991. In this paper, the performance of an electro-coagulation defluoridation batch process with aluminium electrodes was investigated. Different operational conditions such as fluoride concentration in water, pH and current density were varied and performance of the process was examined. Influence of operational conditions on (i) electrode polarization phenomena, (ii) pH evolution during electrolysis and (iii) the amount of aluminium released (coagulant) was investigated. Removal by electrodes is primarily responsible for the high defluoridation efficiency and the adsorption by hydroxide aluminium floc provides secondary effect. Experimental data obtained at optimum conditions that favored simultaneous mixing and flotation confirmed that concentrations lower than 1 mg/L could be achieved when initial concentrations were between 2 and 20 mg/L. pH value was found to be an important parameter that affected fluoride removal significantly. The optimal initial pH range is between 6 and 7 at which effective defluoridation and removal efficiencies over 98% were achieved. Furthermore, experimental results prominently displayed that an increase in current density substantially reduces the treatment duration, but with increased residual aluminium level. The paper focuses on pilot scale defluoridation process optimization along with aluminium leaching and experimental results were compared with a full-scale plant having capacity of 600 liter per batch.     

Enhancement of the conventional anaerobic digestion of sludge: comparison of four different strategies

Anaerobic digestion (AD) is the preferred option to stabilize sludge. However, the rate limiting step of solids hydrolysis makes it worth modifing the conventional mesophilic AD in order to increase the performance of the digester. The main strategies are to introduce a hydrolysis pre-treatment, or to modify the digestion temperature. Among the different pre-treatment alternatives, the thermal hydrolysis (TH) at 170 degrees C for 30 min, and the ultrasounds pre-treatment (US) at 30 kJ/kg TS were selected for the research, while for the non-conventional anaerobic digestion, the thermophilic (TAD) and the two-stage temperature phased AD (TPAD) were considered. Four pilot plants were operated, with the same configuration and size of anaerobic digester (200 L, continuously fed). The biogas results show a general increase compared to the conventional digestion, being the highest production per unit of digester for the process combining the thermal pre-treatment and AD (1.4 L biogas/L digester day compared to the value of 0.26 obtained in conventional digesters). The dewaterability of the digestate became enhanced for processes TH + AD and TPAD when compared with the conventional digestate, while it became worse for processes US + AD and TAD. In all the research lines, the viscosity in the digester was smaller compared to the conventional (which is a key factor for process performance and economics), and both thermal pre-treatment and thermophilic digestion (TAD and TPAD) assure a pathogen free digestate.     

Effects of ORP, recycling rate, and HRT on simultaneous sulfate reduction and sulfur production in expanded granular sludge bed (EGSB) reactors under micro-aerobic conditions for treating molasses distillery wastewater

An expanded granular sludge bed (EGSB) reactor was adopted to incubate the sludge biogranule that could simultaneously achieve sulfate reduction and sulfide reoxidization to elemental sulfur for treating molasses distillery wastewater. The EGSB reactor was operated for 175 days at 35 °C with a pH value of 7.0, chemical oxygen demand (COD) loading rate of 4.8 kg COD/(m3 d), and sulfate loading rate of 0.384 kg SO(4)(2-)/(m3 d). The optimal operation parameters, including the oxidation reduction potential (ORP), recycling rate, and hydraulic retention time (HRT), were established to obtain stable and acceptable removal efficiencies of COD, sulfate, and higher elemental sulfur production. With an ORP of -440 mV, a recycling rate of 300%, and HRT of 15 h, the COD and sulfate removal efficiencies were 73.4 and 61.3%, respectively. The elemental sulfur production ratio reached 30.1% when the elemental sulfur concentration in the effluent was 48.1 mg/L. The performance results were also confirmed by the mass balance calculation of sulfate, sulfide, and elemental sulfur over the EGSB reactor.     

Granulation of Anammox microorganisms in up-flow reactors.

Experimental studies were performed to evaluate the feasibility of granulation of Anammox microorganisms for biomass retention in up-flow reactors. Two experimental studies, one using a 6.4-L lab-scale reactor with synthetic medium and the other using a 200-L pilot-scale reactor with half-nitrified reject water from a sludge digester were conducted. To enhance the granulation process, seed granules from a UASB reactor were added to both experimental reactors. Granulation of Anammox microorganisms was observed using both the synthetic medium and the reject water. The core of a large proportion of Anammox granules retained part of the original seed biomass. The Anammox granules had a slightly lower density than the seed granules from the UASB process, but the size and other physical properties were comparable. The successful granulation of the Anammox microorganisms led to a stable nitrogen removal performance. The maximum nitrogen removal rate of the lab-scale reactor was observed to be 2.9 kg/(m3 x d) after 173 days of operation and that of the pilot-scale reactor was 6.4 kg/(m3 x d) after 12 months of operation.     

Enhanced primary treatment of concentrated black water and kitchen residues within DESAR concept using two types of anaerobic digesters.

Anaerobic digestion of concentrated domestic wastewater streams--black or brown water, and solid fraction of kitchen waste is considered as a core technology in a source separation based sanitation concept (DESAR--decentralised sanitation and reuse). A simple anaerobic digester can be implemented for an enhanced primary treatment or, in some situations, as a main treatment. Two reactor configurations were extensively studied; accumulation system (AC) and UASB septic tank at 15, 20 and 25 degrees C. Due to long retention times in an AC reactor, far stabilisation of treated medium can be accomplished with methanisation up to 60%. The AC systems are the most suitable to apply when the volume of waste to be treated is minimal and when a direct reuse of a treated medium in agriculture is possible. Digested effluent contains both liquid and solids. In a UASB septic tank, efficient separation of solids and liquid is accomplished. The total COD removal was above 80% at 25 degrees C. The effluent contains COD and nutrients, mainly in a soluble form. The frequency of excess sludge removal is low and sludge is well stabilised due to a long accumulation time.     

Treatment of lower load graywater by using a controlled soil natural treatment system.

Experiences of treating graywater by soil natural treatment systems have not been widely reported. In general terms, graywater has a lower concentrations of organic matter; nutrients (e.g. nitrogen), and microorganisms than combined wastewater; therefore, the graywater treatment capacity of soil should be evaluated. In this study, the performance of a natural soil treatment system was evaluated when shower graywater from a house located at Sapporo City, Japan, was treated by a layer of 30 cm of soil from the Hokkaido University Campus. Results showed very high removal efficiencies of chemical oxygen demand (98%) and Kjeldahl nitrogen (95%) at high infiltration rates (214 cm/d), which denotes that natural soil treatment systems could be a good alternative to treat lower load graywater.     

Use of the ADM1 to investigate the effects of acetoclastic methanogen population dynamics on mesophilic digester stability.

The ADM1 was employed to assess the effect of variations in solids hydrolysis and acetoclastic methanogen process characterizations on municipal digester stability relating to excess acetate utilization capacity. First-order single- and dual-pathway hydrolysis rate functions and single and competitive acetoclastic methanogen rate functions were implemented in the ADM1. The acetate capacity number (ACN), defined as the ratio between the maximum acetate utilization rate and the average acetate production rate, was used to index digester instability. Simulations of a single CSTR at steady state indicate a similar ACN can be obtained with a 12-day SRT digester dominated by Methanosarcina sp and a 24-day SRT digester dominated by Methanosaeta sp. An increase in ACN with a decrease in SRT representing Methanosarcina sp. selection was observed for particulate feed loadings from 40 g COD/L to 90 g COD/L. Feeding frequency and dual-pathway hydrolysis were found to have less effect on the ACN than the competitive acetoclastic model structure.     

Thermophilic anaerobic digestion in compact systems: investigations by modern microbiological techniques and mathematical simulation.

Thermophilic anaerobic digestion in compact systems can be an economical and ecological reasonable decentralised process technique, especially for rural areas. Thermophilic process conditions are important for a sufficient removal of pathogens. The high energy demand, however, can make such systems unfavourable in terms of energy costs. This is the case when low concentrated wastewater is treated or the system is operated at low ambient temperatures. In this paper we present experimental results of a compact thermophilic anaerobic system obtained with fluorescent in situ hybridisation (FISH) analysis and mathematical simulation. The system was operated with faecal sludge for a period of 135 days and with a model substrate consisting of forage and cellulose for a period of 60 days. The change in the microbial community due to the two different substrates treated could be well observed by the FISH analysis. The Anaerobic Digestion Model no. 1 (ADM1) was used to evaluate system performance at different temperature conditions. The model was extended to contribute to decreased methanogenic activity at lower temperatures and was used to calculate energy production. A model was developed to calculate the major parts of energy consumed by the digester itself at different temperature conditions. It was demonstrated by the simulation study that a reduction of the process temperature can lead to higher net energy yield. The simulation study additionally showed that the effect of temperature on the energy yield is higher when a substrate is treated with high protein content.     

A new side stream process for easily degradable industrial waste waters to avoid sludge bulking.

A new treatment scheme for the treatment of easily biodegradable industrial waste waters has been developed. The side stream treatment of dairy waste water with the excess sludge from the domestic treatment line of the regional treatment plant Bad V?slau has been operated successfully for a period of three years during which the industrial load stemming from the dairy increased from 800 kg COD/d to 2,500 kg COD/d with peak loads up to 5,000 kg/d. Despite of the increased load to the treatment plant the total aeration tank volume had not been increased. This treatment is performed in an existing aeration tank of the WWTP (V = 1,800 m3) which is now used as contact tank for the combined aeration of dairy waste water and excess sludge from the domestic treatment line (volume aeration tank = 15,000 m3). In this tank the easily degradable substrate from the industrial waste is mainly adsorbed to the biological sludge and after a mechanical dewatering transferred to the anaerobic digester where it yields in an increased gas production. The filtrate of the dewatering process is completely free from biodegradable material and can without danger of bulking be fed to the aeration tank of the domestic treatment line. The new process has proven to be extremely flexible since already now daily peak loads exceeding the design load by more then 60% could be treated in the plant without any problems. Compared to other alternatives for the dairy waste water treatment that were investigated during this study, the new side stream process is very advantageous. No other pre-treatment process for industrial waste water could have been operated under comparable loading conditions without severe operating problems.     

Treatment of landfill leachate by a pilot-scale modified Ludzack-Ettinger and sulfur-utilizing denitrification process.

Nitrogen removal efficiency of a pilot-scale system consisted of Modified Ludzack-Ettinger (MLE) followed by sulfur-utilizing denitrification (SUDNR) process was evaluated with a landfill leachate. For SUDNR, a down-flow mode sulfur packed bed reactor (SPBR) filled with sulfur and limestone particles was used. Although total nitrogen removal efficiency of the MLE process was about 80% at the recycle ratio of 4, effluent contained 350-450 mg/L NO(3-)-N. Up to a loading rate of 1.2 kg NO(3-)-N/m3-day, the SPBR could achieve complete removal of nitrate, and nitrate removal rate was kept to that level even at higher loading rate. When a COD/N ratio of MLE process was maintained at 2 instead of 4, more organics with molecular weight less than 500 were utilized for heterotrophic denitrification although denitrification was not complete with the lack of electron donors. Clogging in the SPBR, mainly by the accumulation of nitrogen gas in the pores, could easily be removed by introducing the effluent in an upward direction for 1 min at 1 hr intervals. The proposed treatment system could achieve nitrate free effluent with a slight increase in chemical cost. Furthermore, depending on further COD removal requirement after biological treatment, the proposed treatment system can be an economical solution.     

Effects of transient temperature increases on odor production from thermophilic anaerobic digestion.

The City of Los Angeles, Bureau of Sanitation, has implemented thermophilic anaerobic sludge digestion at the Hyperion and Terminal Island Treatment Plants (HTP and TITP). A two-stage continuous-batch process was established at HTP, while a single-stage sequencing batch process was established at TITP. This was to evaluate compliance with the Class A pathogen reduction requirements of U.S. EPA 40 CFR Part 503. A rapid increase of the digester temperature at TITP from 57.5 to 65.5 degrees C caused an increase of the volatile fatty acid to alkalinity ratio, a decline in digester performance, and an elevated production of methyl mercaptan and hydrogen sulfide. A rapid increase of the digester temperature at HTP from 54 to 58 degrees C caused an elevated production of methyl mercaptan, but the effect on the volatile fatty acid to alkalinity ratio and digester performance was insignificant. It is likely that these effects observed at TITP and HTP were transient responses to rapid changes in temperature.