Phosphatase activity in anaerobic bioreactors for wastewater treatment

Phosphatase (PO4ase) activity was investigated in continuous and fed-batch anaerobic bioreactors for wastewater treatment. PO4ase levels were high in continuously fed reactors (880-2632 micromol/L/h), compared to a fed-batch reactor (FBR) (540-1249 micromol/L/h). Alkaline and acid PO4ases were present in all the reactors, but in varying magnitudes and total PO4ase activity exhibited a 10-30% variation even at steady-state reactor conditions. The PO4ase activity was not affected by the inorganic phosphate (Pi) level in the reactors, but biomass level and wastewater type, including specific PO4ases (either alkaline or acid), strongly influenced the PO4ase activity in a reactor. Both flocculated and suspended cells produced PO4ase, and 60-65% of the enzyme was cell bound, remaining entrapped in the extracellular matrix and in cell-free form. Batch studies with anaerobic sludge showed a negative correlation between Pi and PO4ase activity. An increase in PO4ase activity was observed under starvation and higher salinity (above 15 g/L). Glucose and propionate (at 10 mM level) induced PO4ase activity, whereas acetate and butyrate (10 mM) addition had no response. This study also reveals that Archaea and bacteria contributed 45% and 55%, respectively, of total PO4ase activity in anaerobic sludge.     

High-rate anaerobic treatment of Fischer-Tropsch wastewater in a packed-bed biofilm reactor

This study investigates the anaerobic treatment of an industrial wastewater from a Fischer-Tropsch (FT) process in a continuous-flow packed-bed biofilm reactor operated under mesophilic conditions (35 °C). The considered synthetic wastewater has an overall chemical oxygen demand (COD) concentration of around 28 g/L, mainly due to alcohols. A gradual increase of the organic load rate (OLR), from 3.4 gCOD/L/d up to 20 gCOD/L/d, was adopted in order to overcome potential inhibitory effects due to long-chain alcohols (>C6). At the highest applied OLR (i.e., 20 gCOD/L/d) and a hydraulic retention time of 1.4 d, the COD removal was 96% with nearly complete conversion of the removed COD into methane. By considering a potential of 200 tCOD/d to be treated, this would correspond to a net production of electric energy of about 8 × 10⁷ kWh/year.During stable reactor operation, a COD balance and batch tests showed that about 80% of the converted COD was directly metabolized through H₂⁻ and acetate-releasing reactions, which proceeded in close syntrophic cooperation with hydrogenotrophic and acetoclastic methanogenesis (contributing to about 33% and 54% of overall methane production, respectively). Finally, energetic considerations indicated that propionic acid oxidation was the metabolic conversion step most dependent on the syntrophic partnership of hydrogenotrophic methanogens and accordingly the most susceptible to variations of the applied OLR or toxicity effects.     

Quantitative detection of previously characterized syntrophic bacteria in anaerobic wastewater treatment systems by sequence-specific rRNA cleavage method

Quantitative monitoring method of two important trophic groups of bacteria in methanogenic communities was established and applied to six different anaerobic processes. The method we employed was based upon our previous sequence-specific rRNA cleavage method that allows quantification of rRNA of target groups so that the populations reflecting in situ activity could be determined. We constructed a set of scissor probes targeting the Chloroflexi group known as ‘semi-syntrophic’ heterotrophic bacteria and fatty acid-oxidizing syntrophs to determine their relative abundance in the processes. By using the method, we found that several reactors harbored a large amount of organisms belonging to the phylum Chloroflexi accounting for up to 20% of the total prokaryotic populations. Propionate-oxidizing syntrophs, Syntrophobacter, Smithella and Pelotomaculum were also found to be significant comprising up to 3.9% of the total populations, but their distribution is highly dependent on the process examined. This is the first clear, non-PCR based quantitative evidence that those organisms play active roles under in situ methanogenic conditions.     

Greenhouse gas production: a comparison between aerobic and anaerobic wastewater treatment technology

Anaerobic wastewater treatment offers improved energy conservation with potential reduction in greenhouse gas emissions. Pitfalls exist in that the methane produced in anaerobic treatment can offset any reductions in carbon dioxide emissions, if it is released to the environment. This paper analyzes greenhouse gas emissions from both aerobic and anaerobic treatment systems, including sludge digestion and the losses of dissolved methane in digested biosolids and process effluents. There exists cross over points, ranging from 300 to 700 mg/L influent wastewater BODu, which are functions of the efficiency of the aerobic treatment system. Anaerobic treatment becomes favorable when treating influents higher in concentrations than the cross over values. A technology to recover dissolved methane would make anaerobic treatment favorable at nearly all influent strengths.     

Electricity assisted anaerobic treatment of salinity wastewater and its effects on microbial communities

High salinity wastewater is often difficult to treat using common anaerobic technologies. Considering that high conductivity of salinity wastewater may enhance electrodes reaction to accelerate the decomposition of volatile fatty acids produced in anaerobic digestion, a pair of electrodes was packed into an anaerobic reactor (R1) with the aim to enhance the treatment of salinity wastewater. With increasing the salt concentration (NaCl) gradually from 0 to 50 g/L in 137 days' operation, COD removal in this reactor under the voltage for the electrodes of 1.2 V was well maintained at 93%, while the COD removal in a reference anaerobic reactor without electrodes (R2) decreased to 53%. When the voltage for R1 was cut off, about 10% COD removal was declined, which was still 30 percentage points higher than that in R2. The electrodes enhanced the biodegradation of volatile fatty acids, especially propionate. Fluorescence in situ hybridization analysis confirmed that the relative abundance of propionate-utilizing bacteria in R1 was significantly higher than that in R2. PCR-DGGE analysis of bacteria and archaea domains indicated that the electric field stimulation effectively enriched salt-adapted microorganisms during the treatment.     

Electrochemical sulfide removal and recovery from paper mill anaerobic treatment effluent

Sulfide can be removed from wastewater and recovered as elemental sulfur using an electrochemical process. Recently, we demonstrated this principle of product recovery on synthetic feeds. Here, we present a lab scale electrochemical reactor continuously removing sulfide from the effluent of an anaerobic treatment process operated on paper mill wastewater. The effluent contained 44 ± 7 mg of sulfide-S L⁻¹. Sulfide was reduced to 8 ± 2 mg-S L⁻¹, at a removal rate of 0.845 ± 0.133 kg-S m⁻³ of total anodic compartment (TAC) d⁻¹. The removed sulfide was recovered (75 ± 4% recovery) as pure concentrated alkaline sulfide/polysulfide solution, from which solid elemental sulfur was obtained. The electrochemical sulfide removal was not affected by different soluble constituents or particulate materials present in the wastewater. However, over time sulfide removal decreased due to biological sulfur reduction using the organics present in the wastewater. Therefore, a periodic switching strategy between anode and cathode was developed. Biofilm formation was avoided as the pH of the cathode solution increased to inhibitory levels during cathodic operation, while still allowing full recovery of the sulfur as end product.     

Fuzzy control of disturbances in a wastewater treatment process

This paper describes a real-time process control scheme to cope with the problem of input disturbances in wastewater treatment processes, based on a fuzzy inferential control system. This can detect the presence of a dangerous input conditions whenever either organic overload or inhibitory/toxic compounds are present in the raw wastewater. Based on this diagnosis, a set of fuzzy rules are implemented to divert the process flow and bring the system back into a safe state. The control system was designed and tested using a pilot plant, to which a toxic disturbance was applied. Its behaviour with and without fuzzy control are compared, and the process reliability improvement in the controlled case is demonstrated.     

Treatment of easily degradable wastewater in a stirred anaerobic sequencing batch biofilm reactor

The main objectives of this study were to evaluate the performance of an anaerobic sequencing batch reactor when subjected to a progressive increase of influent glucose concentration and to estimate the kinetic parameters of glucose degradation. The reactor was initially operated in 8-h cycles, treating glucose in concentrations of 500, 1000 and 2000 mg l(-1). No glucose was detected in the effluent under these three conditions. The reactor showed operating stability when treating a glucose concentration of approximately 500 mg l(-1), with filtered chemical oxygen demand (COD) removal efficiencies varying from 93% to 97%. Operational instability occurred in the operation with glucose concentrations of approximately 1000 and 2000 mg l(-1), caused mainly by a production of extracellular polymeric substances (EPS), which led to hydrodynamic and mass transfer problems in the reactor. The mean volatile acid concentration values in the effluent were approximately 159+/-72 and 374+/-92 mg l(-1), respectively. A first-order model was adjusted to glucose concentration profiles and a modified model, including a residual concentration of substrate, was adjusted to COD temporal profiles. To check the formation of EPS, the reactor was operated in 3-h cycles with concentrations of 500 and 1000 mg l(-1). The purpose of this step was to discover if the production of EPS resulted from the biomass's exposure to a low concentration of substrate over a long period of time. Thus, it was hypothesized that a reduction of the time cycle would also reduce the exposure to low concentrations. However, this hypothesis could not be confirmed because large amounts of EPS were formed already under the first operational condition, using approximately 500 mg l(-1) of glucose in the influent, thus indicating the fallacy of the hypothesis. The production of EPS proved to depend on the organic volumetric load applied to the reactor.     

Involvement of protozoa in anaerobic wastewater treatment process

It is only very rarely recognised in literature that anaerobic reactors may contain protozoa in addition to various bacterial and archeal groups. The role of protozoa in anaerobic degradation was studied in anaerobic continuous stirred tank reactors (CSTR) and batch tests. Anaerobic protozoa, especially the ciliated protozoa, have direct influence on the performance of CSTR at all organic loading rates (1–2 g COD l⁻¹ d⁻¹) and retention times (5–10 days). The studies revealed that chemical oxygen demand (COD) removal is strongly correlated to ciliate density in CSTR fed with oleate (suspended COD) and acetate (soluble COD). There was no significant difference in COD removal between reactors fed suspended COD and those fed soluble COD. However, the diversity and number of ciliates is greater in CSTR fed with particulate feed. The mixed liquor suspended solids (MLSS) representing biomass was significantly lower (16–34%) in CSTR with protozoa. In batch tests, increased COD removal and methane production was observed in sludge having ciliates as compared with sludge without protozoa. Methane production increased linearly with number of ciliates (R²=0.96) in batch tests with protozoa. Direct utilization of COD by flagellates and ciliates was observed in bacteria-suppressed cultures. The technological importance of these results is that reactors with protozoa-rich sludge can enhance the rate of mineralization of complex wastewater, especially wastewater containing particulate COD.     

Submerged anaerobic membrane bioreactor for low-strength wastewater treatment: Effect of HRT and SRT on treatment performance and membrane fouling

Three 6-L submerged anaerobic membrane bioreactors (SAnMBRs) with solids retention times (SRTs) of 30, 60 and infinite days were setup for treating synthetic low-strength wastewater at hydraulic retention times (HRTs) of 12, 10 and 8 h. Total COD removal efficiencies higher than 97% were achieved at all operating conditions. Maximum biogas production rate was 0.056 L CH₄/g MLVSS d at an infinite SRT. A shorter HRT or longer SRT increased biogas production due to increased organic loading rate or enhanced dominancy of methanogenics. A decrease in HRT enhanced growth of biomass and accumulation of soluble microbial products (SMP), which accelerated membrane fouling. A drop in carbohydrate to protein ratio also inversely affected fouling. At 12-h HRT, the effect of SRT on biomass concentration in SAnMBRs was negligible and membrane fouling was controlled by variant surface modification due to different SMP compositions, i.e., higher carbohydrate and protein concentrations in SMP at longer SRT resulted in higher membrane fouling rate. At 8 and 10-h HRTs, infinite SRT in SAnMBR caused highest MLSS and SMP concentrations, which sped up particle deposition and biocake/biofilm development. At longer SRT, lower extracellular polymeric substances reduced flocculation of particulates and particle sizes, further aggravated membrane fouling.