Microfibers in anaerobic digestion: Effect of ozone pretreatment.

Wastewater treatment plants receive significant microplastics, which are eventually discharged into the environment. Previous studies indicated that over 90% of microplastics, especially microfibers from laundry wastewater, are retained in primary sludge. The effect of microfibers from household laundry on anaerobic digestion has yet to be fully understood, which is the objective of the present study. The results in this study showed a positive correlation between methane production and the presence of microfibers. Compared to the control, the methane production increased by 2%, 27% and 43% with 20 mg/L, 100 mg/L and 1000 mg/L microfibers spiked into primary sludge, respectively. The present study suggests that microfibers at 20 mg/L insignificantly affected methane production in controlled anaerobic digestion. In contrast, ozone pretreatment of microfibers enhanced gas production by 12% in the same concentration level. Interestingly, ozone pretreatment at a higher concentration (100 mg/L-1000 mg/L) of microfibers did not affect methane production. SEM/EDX results imply that the ozone pretreatment has changed the surface characteristics of the microfibers, which provide more surface area for adsorption. The significant reduction of soluble phosphorus by 58% indicates that microfibers potentially act as a site for adsorption during anaerobic digestion. Overall, the presence of microfibers had a positive effect on anaerobic digestion. However, this work also indicated that the microfibers were not biodegraded during anaerobic digestion. Therefore, microfibers accumulate on biosolids, potentially affecting the final disposal of microfibers.

Floral waste synthesized silver nanoparticles as sensor for Cr (VI) ion detection.

The presence of heavy metal ions, particularly Cr (VI) in water, is a serious environmental concern. There is a need to develop low-cost and efficient methods for sensing and removing Cr (IV) ions selectively. In this paper, floral waste (FW) extract is used for the synthesis of Ag NPs for sensing Cr (VI) ions in an aqueous solution. Silver nanoparticles (Ag-NPs) were characterized using spectroscopic (UV-visible and FTIR), X-ray diffraction, TEM, and SEM techniques. UV-visible spectroscopic studies revealed that in the presence of Cr (VI) ions, there is an abrupt change in the λmax value of Ag NPs in aqueous solution, indicating that the synthesized Ag-NP is an excellent sensor for the spectroscopical detection of Cr (VI) ions with a low detection limit of 0.5 ppm. The method is fast, economical, simple, and efficient.

Effect of ferrate pretreatment on anaerobic digestibility of primary sludge spiked with resin acids.

Resin acids are mixtures of high molecular weight carboxylic acids found in tree resins. Due to higher hydrophobicity and low solubility, they tend to adsorb on the suspended solids in pulp and paper (P&P) mill wastewater and accumulate in primary sludge through settling. Anaerobic digestion (AD) is a common practice stabilizing sludge; however, high concentration of resin acids affects the AD process. The aim of this research was mainly to determine the impact of ferrate (Fe (VI)) oxidation on selected resin acids and anaerobic digestibility of ferrate-treated primary sludge (PS) spiked with the resin acids. First, batch control oxidation of model resin acids with Fe (VI) was conducted to identify an optimum dosage, pH and contact time using a Box-Behnken design approach. Thereafter, anaerobic treatability studies of primary sludge spiked with resin acids both under control condition and optimum ferrate pretreatment were conducted. Up to 97% oxidation of resin acids occurred in pure water, while only 44%-62% oxidation of resin acids occurred in PS with an increasing Fe (VI) dosage from 0.034 to 0.137 mg Fe (VI)/mg tCODfed. The pretreatment did not affect the anaerobic biodegradability of resin acids; however, it lowered their negative influences on the PS digestibility. About 0.076 mg Fe (VI) dosage/mg tCODfed solubilized the sludge increasing the methane production by 40% compared to the untreated digester. The potential benefits of ferrate pretreatment of P&P primary sludge include resin acids oxidation and subsequent toxicity reduction, higher sludge solubilization enhancing methane production and enabling anaerobic digestion at higher COD loading.

Effect of coagulation on microfibers in laundry wastewater.

Microplastics pollution in the aquatic system has received significant attention due to their recalcitrant nature and ecotoxicological threat. Municipal wastewater typically contains various microplastics with synthetic microfibers as a significant constituent from the laundry process. The fate of microfibers in conventional wastewater processes is not clearly understood. In this study, the effect of coagulation on microfibers obtained from a lint screen of a domestic dryer and resuspended in pure water, and also in laundry wastewater was investigated using ferric chloride and polyaluminum chloride (PACl). The removal efficiency of the microfibers resuspended in pure water varied from 86% to 96% depending on the fiber size ranges: < 90 µm, 90-125 µm, and >125 µm with the smaller size microfibers showing a lower removal efficiency. Surfactant present in detergent in laundry wastewater reduced the microfibers removal efficiency to 0-37%, however, the addition of PACl increased microfibers removal to 90%. The optimal PACl concentrations for ≥90% removal were 1.75, 2, 4, and 6 mg/L for 0.5, 2, 4, 8 mg/L detergent, respectively. Zeta potential, FTIR, and SEM analysis were applied to observe the surface changes of microfibers during coagulation indicating possible mechanisms of coagulation. The dominant mechanisms for coagulation of microfibers by FeCl3 and PACl seem to be charge neutralization and adsorption-bridging. This work provided some insights about the fate of laundry microfibers in primary treatment processes.

Adsorption isotherms and kinetics for the removal of algal organic matter by granular activated carbon.

Seasonal algal blooms in surface water release a significant amount of algal organic matter (AOM), which alters the composition of dissolved organic matter (DOM). AOM affects the drinking water treatment processes and finished water quality. In this study, the relative removal efficiency of AOM and humic acid by granular activated carbon (GAC) adsorption was determined. Batch experiments were conducted to evaluate the adsorption capacity of GAC, which varied from 4.235-31.45 mg/g for AOM originated from different algae. Freundlich isotherm models fitted the adsorption equilibrium data, and the adsorption kinetics data were fitted well using a pseudo-second order kinetic model. The calculated thermodynamics parameters (∆G0, ∆H0 and ∆S0) indicated that GAC adsorption for DOM removal was endothermic and spontaneous in nature.

Anaerobic digestibility of resin acids in primary sludge: Effect of ozone pretreatment.

Resin acids in pulp and paper mills wastewater are potentially partitioned in the solids in post-primary clarification due to higher hydrophobicity with log Kow ∼1.74-5.80. They are known to adversely affect anaerobic digestion (AD) process, although the effect has not been quantified deterministically in control studies. The objective of the present work was to determine the effect of untreated and ozonated spiked resin acids on AD of primary sludge. Batch adsorption tests were conducted to determine the solid-liquid partition coefficient (Kd) of resin acids on the primary sludge. Higher Kd was obtained at pH 4; however, it was decreased by 78-98% at pH 8. Thereafter, batch AD of model resin acids in primary sludge using food to microorganism ratio (S0/X) of 0.5gtCOD/gVSSindicated only 15-20% removal of resin acids in the liquid phase anaerobically. While, ozonation in pure water using 0.74-1.48 mg O3/mg tCOD showed >90% reduction of the test resin acids, an ozone dose of 0.52 mg O3/mg tCOD reduced 50-70% spiked resin acids' load to the digester. However, no further removal of resin acids occurred during AD over 30 days. About 42% reduction in methane production compared to the control digestor occurred in the presence of 150 mg/L of resin acids. When treated with 0.52 mg O3/mg tCOD, methane production improved and was comparable to the control digestor, indicating that resin acids may not be detrimental to AD at a concentration range of 45-75 mg/L.

Simultaneous quantification of five pharmaceuticals and personal care products in biosolids and their fate in thermo-alkaline treatment.

The presence of pharmaceuticals and personal care products (PPCPs) in biosolids applied to farmland is of concern due to their potential accumulation in the environment and the subsequent effects on humans. Thermo-alkaline hydrolysis (TAH) is a method used for greater stabilization of biosolids after anaerobic digestion. In this work, the effect of TAH on five selected PPCPs including fluoroquinolone antibiotics, ciprofloxacin (CIP), and ofloxacin (OFLX), and three commonly used antimicrobial agents, miconazole (MIC), triclosan (TCS) and triclocarban (TCC) was evaluated. At the onset, extraction and analytical methods were optimized for maximum simultaneous recovery and LC-MS quantification of the target PPCPs from both water and biosolids for improved accuracy. The compounds were detected in the range of 54 ± 3 to 6166 ± 532 ng/g in raw biosoilds collected from a local WWTP. Next, batch control adsorption experiments of the selected PPCPs were conducted in various sludges, which indicated about 89%-98% sorption of the PPCPs onto solid phase due to their high octanol-water coefficients. Subsequently, thermo-alkaline (pH 9.5, 75 °C, 45 min) hydrolysis (TAH) was conducted to determine the extent of degradation of these compounds in deionized (DI) water and biosolids due to treatment. The degradation of these compounds due to TAH ranged from 42% to 99% and 37%-41% in pure water and biosolids, respectively, potentially lowering their risk in the environment due to land application. A list of compounds for which the optimized analytical method potentially can be used for detection and quantification in environmental samples is provided in the supporting document.

Fate of micropollutants in chemically enhanced primary treatment using recovered coagulants.

In this study, the fate of several micropollutants (MPs) in wastewater due to coagulation using both fresh and recovered aluminum and iron coagulants was determined. 18 MPs from different groups such as antibiotics, food additives, and surfactants were selected and spiked into the primary influent collected from a local wastewater plant. The distribution of MPs in the recovered coagulant and treated effluent after coagulation was determined for both fresh and recycled coagulants. The distribution of MPs in wastewater and the removal during coagulation were compound specific; MPs with log Kow < 2.5 were predominantly present in the effluent after coagulation, while MPs with log Kow > 2.5 were sorbed on the coagulated sludge. The distribution ratio (Kd) of all the MPs (diclofenac, clarithromycin, etc.) with log Kow > 2.5 was determined along with their extent of accumulation in sludge due to the recycling of coagulants. Compounds such as sulfamethoxazole, erythromycin and sulfathiazole, showed low removal during coagulation. The tetracycline group of compounds showed possible chelation with iron and aluminum. Only <10% of the initially spiked MPs with log Kow > 2.5 was being recycled with the recovered coagulant, thus alleviating the concern of accumulation of the MPs during recycle of the coagulants.

Coagulation and disinfection by-products formation potential of extracellular and intracellular matter of algae and cyanobacteria.

Coagulation and flocculation can remove particulate algal cells effectively; however, they are not very effective for removing dissolved algal organic matter (AOM) in drinking water plants. In this work, optimum coagulation conditions using alum for both extracellular and intracellular organic matter of six different algal and cyanobacterial species were determined. Different coagulation conditions such as alum dosage, pH, and initial dissolved organic carbon (DOC) were tested. Hydrophobicity, hydrophilicty, and transphilicity of the cellular materials were determined using resin fractionation method. The removal of DOC by coagulation correlated well with the hydrophobicity of the AOM. The disinfection by-product formation potential (DBPFP) of various fractions of AOM was determined after coagulation. Although, higher removal occurred for hydrophobic AOM during coagulation, specific DBPFP, which varied from 10 to 147 µg/mg-C was higher for hydrophobic AOM. Of all the six species, highest DBPFP occurred for Phaeodactylum tricornutum, an abundant marine diatom species, but is increasingly found in surface water.

Effect of COD/N ratio on denitrification from nitrite.

The objective of this study was to investigate dynamic specific denitrification rates (SDNRs) from nitrite at various chemical oxygen demand (COD)/nitrogen (N) ratios using municipal wastewater (MWW). A sequencing batch reactor (SBR) continuously fed with primary effluent and nitrite solution was operated at hydraulic retention time of 8.4 hr and solids retention time of 26-30 days for 3 months. Influent MWW characteristics varied significantly during the study, that is, 200-810 mgCOD/L and 6-80 mgN/L. The SDNRs from the SBR were compared with those determined in four batch reactors using acetate. The SDNR was directly related to COD/N until a maximum SDNR (mgNO2 -N/mgVSS/day) of 0.07 for MWW and 0.4 for acetate occurred at COD/N ratios of 6 and 13, respectively; beyond this COD/N ratio, SDNR decreased. The biomass yield coefficients (mgVSS/mgCOD) were 0.33 for MWW and 0.51 for acetate. The relationships of SDNR with COD/N and F/M ratios were developed. PRACTITIONER POINTS: The optimum carbon dose for denitrification should be determined using acclimatized biomass. Each carbon source should only be dosed at an optimum that maximizes denitrification.

Continuous column adsorption of naphthenic acids from synthetic and real oil sands process-affected water (OSPW) using carbon-based adsorbents.

In this study, activated petroleum coke (APC) and commercial activated carbon (CAC) were used in a continuous adsorption column for removal of model naphthenic acids and organics from real oil sands process-affected water (OSPW). Diphenylacetic acid and 2-naphthoic acid, two model naphthenic acid (NA) compounds, were removed completely by the APC in a continuous column operation. Due to the complex nature of organics in OSPW, total organic carbon (TOC) was measured to determine the effectiveness of OSPW treatment by APC. The removal of TOC from OSPW at its natural pH 8 by APC was only 25%, whereas acidification at pH 4 followed by APC adsorption removed 96% of the initial TOC. When compared to a commercial activated carbon, the APC showed an average of 20% higher organics removal. The experimental breakthrough curves were better fitted by Thomas model in comparison to Adams-Bohart and Yoon-Nelson models. The regeneration of APC was conducted using methanol with 0.01 wt% NaOH (pH = 11.7) and a total of four cycles of adsorption and regeneration were conducted with marginal loss of adsorption sites.

Assessment of water samples with complex compositions using microalgal bioassay based on the community level physiological profiling (CLPP).

The ability to effectively characterize the response of microalgal communities to changes in water quality is limited. Earlier, a microalgal bioassay was developed based on community level physiological profiling (CLPP). The efficacy of this assay was evaluated using three wetland water samples, a surface water sample, and two wastewater samples (i.e. primary and secondary), all collected from southwestern Ontario, Canada. In addition, the assay was applied to untreated and activated carbon treated oil sand process water (OSPW). YT (Yeast Identification Test Panel) and Biolog plates were successfully utilized for defined microalgal community under both heterotrophic and mixotrophic growth conditions to characterize the changes in the defined microalgal community due to the changes in water type. It was found that, although the degrees of changes in the algal community varied, all tested water samples were distinguished under both growth regimes using principal component analysis (PCA). The variations in the algal community were caused by the differences of the water samples. The response of the assay due to changes in the algal community caused by different waters was found to be very sensitive and could be used to differentiate different water bodies. It further can be used to monitor temporal changes of water quality of the same water body.

Preparation of activated petroleum coke for removal of naphthenic acids model compounds: Box-Behnken design optimization of KOH activation process.

This study employed Box-Behnken design and response surface methodology to optimize activation parameters for the production of activated petroleum coke (APC) adsorbent from petroleum coke (PC) to achieve highest adsorption capacity for three model naphthenic acids. Activated petroleum coke (APC) adsorbent with a BET surface area of 1726 m2/g and total pore volume of 0.85 cc/g was produced at the optimum activation conditions (KOH/coke mass ratio) of 3.0, activation temperature 790 °C, and activation time 3.47 h). Effects of the activation parameters on the adsorption pefromances (adsortion capaciy and kinetics) were investigated. With the APC obtained at the optimum activation condition, the maximum adsorption capacity of 451, 362, and 320 (mg/g) was achieved for 2-naphthoic acid, diphenylacetic acid and cyclohexanepentanoic acid (CP), respectively. Although, generally APC adsorbents with a higher specific surface area and pore volume provide better adsorption capacity, the textural properties (surface areas and pore volume) are not the only parameters determining the APC adsorbents' adsorption capacity. Other parameters such as surface functionalities play effective roles on the adsorption capacity of the produced APC adsorbents for NAs. The KOH activation process, in particular the acid washing step, distinctly reduced the sulfur and metals contents in the raw PC, decreasing the leaching potential of metals from APC adsorbents during adsorption.

Uptake and phytotoxic effect of benzalkonium chlorides in Lepidium sativum and Lactuca sativa.

Cationic surfactants such as benzalkonium chlorides (BACs) are used extensively as biocides in hospitals, food processing industries, and personal care products. BACs have the potential to reach the rooting zone of crop plants and BACs might thereby enter the food chain. The two most commonly used BACs, benzyl dimethyl dodecyl ammonium chloride (BDDA) and benzyl dimethyl tetradecyl ammonium chloride (BDTA), were tested in a hydroponic system to assess the uptake by and phytotoxicity to lettuce (Lactuca sativa L.) and garden cress (Lepidium sativum L.). Individually and in mixture, BACs at concentrations up to 100 mg L-1 did not affect germination; however, emergent seedlings were sensitive at 1 mg L-1 for lettuce and 5 mg L-1 for garden cress. After 12 d exposure to 0.25 mg L-1 BACs, plant dry weight was reduced by 68% for lettuce and 75% for garden cress, and symptoms of toxicity (necrosis, chlorosis, wilting, etc.) were visible. High performance liquid chromatography-mass spectroscopy analysis showed the presence of BACs in the roots and shoots of both plant species. Although no conclusive relationship was established between the concentrations of six macro- or six micro-nutrients, growth inhibition or BAC uptake, N and Mg concentrations in BAC-treated lettuce were 50% lower than that of control, indicating that BACs might induce nutrient deficiency. Although bioavailability of a compound in hydroponics is significantly higher than that in soil, these results confirm the potential of BACs to harm vascular plants.

Carbon and Phosphorus Removal from Primary Municipal Wastewater Using Recovered Aluminum.

In this work, recovery of aluminum from coagulated primary sludge and its reuse potential as secondary coagulant were investigated. The recovery process consisted of releasing the particle-bound aluminum from primary sludge by acidification (HCl or H2SO4), followed by separation using centrifugation for dissolved coagulant recovery. The recovered coagulant was then reused for treating primary wastewater and overall coagulation efficiency was determined. While with fresh alum, the removal efficiencies of total suspended solids, chemical oxygen demand, total phosphorus, and total nitrogen were 85%, 65%, 80% and 33%, respectively, a drop in removal efficiency of total suspended solids and chemical oxygen demand was observed for recovered aluminum (85-60% and 65-50%, respectively). Nitrogen concentration remained almost constant with each cycle, while phosphorus in the effluent increased by 1 mg/L and 3 mg/L in the first and second cycle, respectively. Precipitation of various aluminum species was modeled for determining the recovery potential of aluminum at low pH. Preliminary cost analysis indicates that optimum recovery of aluminum occurred at a pH of 1.5 for both acids. Struvite precipitation effectively removed increased phosphorus solubilized by acidification at the end of second cycle, however, it also decreased the amount of aluminum available for recycle.

Sorption and leaching of benzalkonium chlorides in agricultural soils.

The adsorption and leaching characteristics of two commonly used benzalkonium chlorides (BACs), benzyl dimethyl dodecyl ammonium chloride (BDDA) and benzyl dimethyl tetradecyl ammonium chloride (BDTA) using three agricultural soils with varied proportions of silt, sand, clay, and organic matter were determined. BACs are cationic surfactants used in large quantities for sanitary and personal care products and are abundant in environmental samples. Adsorption isotherm data (aqueous concentration in the range of 25-150 mg L-1) fitted the Langmuir model better than the Freundlich model. BDTA with a longer alkyl chain adsorbed more to soil compared to BDDA, and the soil with the highest percentage of clay adsorbed the most. Column tests conducted using soils amended with lime stabilised biosolids and artificial rain water at a flow rate of 0.2 mL min-1 indicate very low leaching of BACs. Less than 1% of the available BDDA leached through sandy loam soil column with a depth of 9 cm. Therefore, the possibility of BACs to become bioavailable through leaching is very low at environmentally relevant concentrations.

Low-temperature thermal pre-treatment of municipal wastewater sludge: Process optimization and effects on solubilization and anaerobic degradation.

The present study examines the relationship between the degree of solubilization and biodegradability of wastewater sludge in anaerobic digestion as a result of low-temperature thermal pre-treatment. The main effect of thermal pre-treatment is the disintegration of cell membranes and thus solubilization of organic compounds. There is an established correlation between chemical oxygen demand (COD) solubilization and temperature of thermal pre-treatment, but results of thermal pre-treatment in terms of biodegradability are not well understood. Aiming to determine the impact of low temperature treatments on biogas production, the thermal pre-treatment process was first optimized based on an experimental design study on waste activated sludge in batch mode. The optimum temperature, reaction time and pH of the process were determined to be 80 °C, 5 h and pH 10, respectively. All three factors had a strong individual effect (p < 0.001), with a significant interaction effect for temp. pH2 (p = 0.002). Thermal pre-treatments, carried out on seven different municipal wastewater sludges at the above optimum operating conditions, produced increased COD solubilization of 18.3 ± 7.5% and VSS reduction of 27.7 ± 12.3% compared to the untreated sludges. The solubilization of proteins was significantly higher than carbohydrates. Methane produced in biochemical methane potential (BMP) tests, indicated initial higher rates (p = 0.0013) for the thermally treated samples (khyd up to 5 times higher), although the ultimate methane yields were not significantly affected by the treatment.

Biodegradation of benzalkonium chlorides singly and in mixtures by a Pseudomonas sp. isolated from returned activated sludge.

Bactericidal cationic surfactants such as quaternary ammonium compounds (QACs) are widely detected in the environment, and found at mg kg(-1) concentrations in biosolids. Although individual QACs are amenable to biodegradation, it is possible that persistence is increased for mixtures of QACs with varying structure. The present study evaluated the biodegradation of benzyl dimethyl dodecyl ammonium chloride (BDDA) singly and in the presence of benzyl dimethyl tetradecyl ammonium chloride (BDTA) using Pseudomonas sp., isolated from returned activated sludge. Growth was evaluated, as was biodegradation using (14)C and HPLC-MS methods. BDTA was more toxic to growth of Pseudomonas sp. compared to BDDA, and BDTA inhibited BDDA biodegradation. The benzyl ring of [U-(14)C-benzyl] BDDA was readily and completely mineralized. The detection of the transformation products benzyl methyl amine and dodecyl dimethyl amine in spent culture liquid was consistent with literature. Overall, this study demonstrates the antagonistic effect of interactions on biodegradation of two widely used QACs suggesting further investigation on the degradation of mixture of QACs in wastewater effluents and biosolids.

Anaerobic digestibility of estrogens in wastewater sludge: effect of ultrasonic pretreatment.

BACKGROUND: Estrogenic compounds have been detected in the secondary effluents and in the biosolids from conventional wastewater treatment plants, which are not designed for their removal. Furthermore, existing limited studies on anaerobic digestibility of estrogens report conflicting results. The objective of the present work was to determine the fate and anaerobic digestibility of estrogenic compounds in various types of sludge including primary sludge (PS), waste activated sludge (WAS), and anaerobically digested sludge (seed). METHODS: Estrone (E1) and 17-ß estradiol (E2) were chosen as the model estrogenic compounds. Initially batch adsorption was conducted to determine the extent of adsorption and isotherm of E1 on various sludge. Thereafter, batch anaerobic digestion of E1 and E2 was conducted in various sludge using So/X ratio of 4 gCOD/gVSS in 250 ml bottles. The effect of earlier optimized ultrasonication dosage on the anaerobic digestion of E1 and E2 was also characterized. Estrogenicity of the digested samples was determined by the YES assay. RESULTS: Most of E1 and E2 was adsorbed on the biosolids and the Freundlich isotherm fitted the experimental data well. No anaerobic digestion of E1 and E2 was found in any of the sludge tested, and the estrogenicity of the sludge measured by YES assay increased during digestion due to the formation of E2 from E1 in a reduced environment. Ultrasonication decreased the initial mass of E1 and E2 by 20% in the sonicated digester as compared to control digester, however, there was no further decrease in E1 and E2 during digestion. CONCLUSIONS: Most of the estrogenic compounds partitioned onto the solids and remained there during digestion. Ultrasonication pretreatment reduced the estrogen burden for the digester due to advance oxidation, but no further removal of the estrogens occurred in the digester.

Degradation of estrone in water and wastewater by various advanced oxidation processes.

A comprehensive study was conducted to determine the relative efficacy of various advanced oxidation processes such as O3, H2O2, UV, and combinations of UV/O3, UV/H2O2 for the removal of estrone (E1) from pure water and secondary effluent. In addition to the parent compound (E1) removal, performance of the advanced oxidation processes was characterized using removal of total organic carbon (TOC), and estrogenicity of the effluent. Although E1 removal was high for all the AOPs, intermediates formed were more difficult to degrade leading to slow TOC removal. Energy calculations and cost analysis indicated that, although UV processes have low electricity cost, ozonation is the least cost option ($ 0.34/1000 gallons) when both capital and operating costs were taken into account. Ozonation also is superior to the other tested AOPs due to higher removal of TOC and estrogenicity. The rate of E1 removal decreased linearly with the background TOC in water, however, E1 degradation in the secondary effluent from a local wastewater treatment plant was not affected significantly due to the low COD values in the effluent.