Removal of organics and metal ion nanoparticles from synthetic wastewater by Activated Sludge Process (ASP)

Abstract— Adsorption technique is widely used for removal of toxic organic contaminants from aqueous streams. Owing to the hazardous or otherwise undesirable characteristics of phenolic compounds in particular, their presence in wastewater from municipal and industrial discharge is one of the most important environmental issue. The discharge of poor quality effluents by the chemicalbased laboratories and refineries in India is posing a serious threat to water sources and wastewater treatment installations alike. Our study was set up in the Indo-French Unit for Water & Wastewater Technologies (IFUWWT), IIT Delhi. The main objective of this study was to assess the efficiency of a laboratory-scale activated sludge treatment process in producing a final effluent conforming to regulatory standards of Central Pollution Control Board, India with regards to COD and metal ion loads. The study was conducted in three principal stages: characterization of wastewater containing nanoparticles; treatability studies of laboratory generated discards and investigations of heavy metal ions before and after treatment. The raw effluent parameters analyzed were COD, BOD, F/M ratio, Sludge Value Index, Total Solids and concentrations of Cu, Ag and Zn.


INTRODUCTION
In many arid and semi-arid countries water is becoming an increasingly scarce resource and planners are forced to consider any sources of water which might be used economically and effectively to promote further development. The wastewater that is generated by the laboratories is characteristically high in both organic and inorganic content. The ability to reclaim wastewater for discharge or reuse would be a giant step toward over-all waste reduction.
The conventional methods for treatment of effluents contaminated with heavy metals involve physicochemical processes such as flocculation, precipitation, electrolysis and crystallization. However, these processes are very expensive and generate new products, merely resulting in a transfer of the metal from one medium to another, but not providing a definitive solution. The search for cheaper and definitive solutions led to the development of new technologies based on the utilization of organic substrates for removal of heavy metals by the process of sorption using bioreactors.
A bioreactor (BR) may refer to any engineered device or system that supports a biologically active environment. The aim of the study was to set up a vessel in which a chemical process can be carried out, which involves organisms or biochemically substances derived from such organisms. The sludge used was obtained from Vasant Kunj wastewater treatment plant. To keep the process aerobic, the sludge was kept on aeration throughout. This process is functioned to treat waste water using bacteria which is helped by its food. www.ijergs.org

CHARACTERIZATION OF CARBOY NANOPARTICLES (NPs)
In order to design onsite wastewater treatment systems, we must consider the nature of the wastewater because the effluent quality depends upon the influent characteristics. The treatment capacity and treatment efficiency of systems are calculated based upon the influent concentrations and the effluent requirements.
The source of the wastewater influences the characteristics of the waste stream. In general, we can categorize the source as residential, municipal, commercial, industrial or agricultural. The sample for the purpose of our study was a carboy whose constituents were the discarded materials of our laboratory. The components of influent-characterization would be: a. TS (Total Solid) Suspended Solid (SS) parameter/Non-Filterable Residue refers to the dry weight of particles trapped by the Whattman Filter Paper of 45 micron pore size while Dissolved Solid (DS) refers to the dry mass left behind on the filter paper. Their summation gives the net Total Solid content of the waste-water.
b. BOD 5 According to CPCB, the maximum permissible limit of suspended solids for irrigational land is 200mg/L while that for inland surface water is 100mg/L. CPCB guidelines state that those water bodies having BOD more than 6 mg/l are identified as polluted water bodies.
Biological oxygen demand (BOD) is a measure of the amount of oxygen that is consumed by bacteria during the decomposition of organic matter. Having a safe BOD level in wastewater is essential to producing quality effluent. If the BOD level is too high then the water could be at risk for further contamination, interfering with the treatment process and affecting the end product. By comparing the BOD of incoming sewage and the BOD of the effluent water leaving the plant, the efficiency and effectiveness of sewage treatment can be judged.

c. COD
The COD (Chemical Oxygen Demand) test represents the amount of chemically digestible organics (food). COD measures all organics that were biochemically digestible as well as all the organics that can be digested by heat and sulfuric acid.
For our purpose, COD was determined using Closed Reflux method ( (APHA, 1989); (González, 1986); (Jirka & Carter, 1975)). In this case, a small volume of sample is heated with concentrated dichromate solution in presence of silver sulphate and mercuric sulphate. The reaction takes place in culture tubes with PTTE-lined screw caps at 150° C. Heating proceeds for usually shorter times, at higher temperatures than in the open reflux method; the COD is estimated by titrating the digested sample against ferrous ammonium sulphate solution (FAS) in the presence of Ferroin indicator. d. Mixed Liquor Volatile Suspended Solid (MLVSS) e. pH f. Calcium and Magnesium content (to check for hardness) g. Metal Analysis

DETERMINATION OF TRACE METAL IONS BY ATOMIC ABSORPTION SPECTROMETER
This is done after pre-concentration and subsequent concentration reduction on adsorption by sludge Samples Tested for presence of heavy metal ions: Our aim was to characterize the carboy nanoparticle sample taking the concentration of three heavy metal ions, namely Cu, Ag and Zn, into consideration. Furthermore, we needed to check if the process of sludge-based-adsorption can prove to be an effective measure to remove these hazardous heavy metals. Hence the metal analyses were done for (a) carboy (b) sludge and supernatant for both reactors. www.ijergs.org Sample Digestion: To ensure the removal of organic impurities from the samples and thus prevent their interference in analysis, the samples were digested with concentrated nitric acid. 10ml of nitric acid was added to 50ml of sample to be analyzed in a 250ml conical flask. The mixture was evaporated to volume of 5mL by keeping it over a heating plate chamber after which it was allowed to cool and then filtered. Sample Analysis: The digested water samples were analyzed for the presence of copper, silver and zinc using the ElementAS AAS4141 Atomic Absorption Spectrophotometer (by Electronics Corporation of India Ltd). The calibration plot method was used for the analysis. Air-acetylene was the flame used and hollow cathode lamp of the corresponding elements was the resonance line source, the wavelength for the determination of the elements were 217.9nm, 327.5nm and 212.6nm for copper, silver and zinc respectively. The digested samples were analyzed in duplicates with the average concentration of the metal present being displayed in mg/L by the instrument after extrapolation from the standard curve.

a. BIOLOGICAL REACTOR SETUP
The sludge which provided biomass was obtained from Vasant Kunj Wastewater Treatment plant. To prepare the feed/food for biomass-generation, 1 gram of D-Glucose was added to a liter of water kept on aeration for roughly an hour. This served the purpose for influent COD. 1ml each of MgSO 4 .7H 2 O, CaCl 2 , FeCl 3 .6H 2 O and phosphate buffer was next added to the vessel. 1 gram of glucose corresponds to a COD of 1066.67 mg/L of O 2 . This solution was added by an amount equal to the supernatant decanted from each reactor. The reactors were kept on aeration overnight. Next day, the COD of the supernatant/effluent would be calculated using the closed-reflux-method. The biomass used wasn't discarded throughout the duration of the study.

b. RUNNING ADSORPTION EXPERIMENTS TO STABILIZE COD AND BOD 5
The overall goal of the activated sludge was to reduce or remove organic matter, solids, ions nutrients, and other pollutants from wastewater. More specifically, the activated sludge process involved blending settled primary effluent wastewater with a culture of microorganisms into fluid called -mixed liquor‖. The mixed liquor was discharged in an activation tank, in which air was introduced into the system to create an aerobic environment that kept the activated sludge properly mixed. The bacteria stabilized the substances that had a demand for BOD, while oxidizable chemicals (reducing chemicals) were responsible for consuming COD before being discharged in a clarifier where suspended solids and liquid were separated.
Oxidizable material + bacteria + nutrient + O2  CO2 + H2O + oxidized inorganics A number of treatment technologies are in use for the treatment of wastewater contaminated with organic substances. Among them, adsorption process is considered as a promising method for removing COD, heavy metals, colour, odour and ions. This method has aroused considerable interest during recent years for cleaning the wastewater specifically due to its cost-effectiveness. Adsorption in Bioreactor B had been carried out in two stages. For an initial length of 20 days, only glucose-based organic feed was added to the reactor. The aim was to acclimatize the microorganisms residing in the reactor to glucose, which served as food for the microorganisms. After this initial set of 20 days, it was seen that the percentage reduction in COD didn't witness any changes. Hence, it was concluded that the bacteria had become acclimatized. Carboy nanoparticles were now introduced alongside the glucose to the bioreactor B. The ratio for carboy sample to glucose feed was kept at 1:1. This step was prompted due to small concentration of metal ions and had the ratio been kept smaller, the difference between CODs of mixed liquor and Bioreactor A would have been too small. The setup was studied for duration of one week. The BOD 5 during our study came out to be 697.9949mg/L. The high content of microorganisms and other organic matter lead to consumption of the available oxygen.

a. CHARACTERIZATION DATA OF CARBOY NANOPARTICLES (NPs)
For the purpose of experiment, we had proceeded with COD monitoring instead of BOD 5 because of following reasons: BOD 5 /COD ratio for June 3, 2013 was 0.373; the result indicated that COD readings are significantly greater than those of BOD 5 . Secondly, for COD calculation, the sample needs to be kept in the digestor for only 2 hours. BOD calculation requires duration of 5 days, hence being inconvenient. By the time we have results from a 5-day test; the plant conditions would no longer be same. Hence, real time monitor and control cannot be relied upon BOD. After June 6, a certain amount of laboratory discarded waste was again feeded to the aging carboy to regenerate it. This explains the visible bump in COD readings. The constant decrement in the COD values can be attributed to decomposition of organic wastes and oxidation of chemical waste.
In accordance with the (CPCB, 2007) guidelines, the maximum permissible Chemical Oxygen Demand of Inland Surface Water could be 250mg/L and for drinking purposes, it comes down to 3 mg/L. With respect to these values, the given carboy sample can be categorized to be highly contaminated as its initial COD value is over 1500 mg/L. This result was expected as the constituents of the carboy are formed by laboratory-discards. Before examining metal-ion analysis, it is important to understand the primary role of activated sludge i.e. the reduction of chemical oxygen demand (COD) from domestic wastewater.

COD removal and sludge yield
Simply due to the high number of microorganism in bioreactors, the pollutants uptake rate can be increased. This leads to better degradation in a given time span; also the required reactor volumes are smaller. In comparison to the conventional activated sludge process (ASP (Trussel, 2006)) which typically achieves 95 percent removal, average COD removal by Reactor-A over a course of 30 days came out to be 98.186%. Such a high sludge yield could be attributed to high MLSS concentration.
F/M ratio and dissolved oxygen (DO) concentration has a big influence to on microorganism growth in activated sludge process. A rapid growth causes bulking sludge which is indicated by a High SVI value.

Calculating F/M ratio
The term Food to Microorganism Ratio i.e. F/M ratio, as per (DEP), is actually a measurement of the amount of incoming food (kg of Influent CBOD) divided by the kg of microorganisms in your system. In our calculations, the volume of activated sludge in our clarifiers has been taken as the total amount of microorganisms exposed to the incoming food.
Volume of supernatant removed everyday = 500ml/L ⇒ Volume of microbial-sludge exposed = 500ml/L   Normally, we prefer a low F/M ratio due to the following advantages: (i) High degree of elimination of BOD5 and COD (ii) Good nitrification/de-nitrification (iii) Good settle-ability to sustain shock and toxic loading.
Extended aeration processes generally operate within the following ranges:  The abnormal reductions in removing capacity towards the end can a result of overburdening of the sludge biomass, indicating the need to replace the sludge.      It is evident that the domestic wastewater remains polluted with organic load plus the dissolved and suspended matter. Organic load is reflected in terms of the COD and the BOD values. In the present investigations only reduction of the COD was discussed. The COD concentrations, 739mg/L to 1014mg/L, in the wastewater were substantially higher than that of the permissible limit: 100-200 mg/l, for irrigation and horticultural uses, according to the Central Pollution Control Board, India (CPCB norms). Silver nanoparticles were found to be absent from carboy and glucose solution. Concentration of copper nanoparticles in the carboy was found out to be 0.064 mg/L. This reading is very insignificant when compared to CPCB parameters.  Another set of readings suggest that when on glucose-based feed was added to the Bioreactor-B, no notable changes were noticed in metal ion's concentration of both supernatant and sludge.

CONCLUSIONS
Calibration curves were obtained using a series of varying concentrations of the standards for both the metals. The two calibration curves were linear with correlation coefficients of 0.996 and 0.9811. The initial biosorption capacities were 95.35% and 100%. The subsequent sludge yield continues to be very high owing to aging of sludge and the fact that we are dealing with heavy metals having very low concentrations. Because of these insignificant values, the amount of metal ions introduced to the system gets adsorbed almost completely, hence leaving behind no metal ion within the supernatant.
For the protection of human health, guidelines for the presence of heavy metals in water have been set by different International Organizations such as (USEPA), (WHO) and European Union. Thus, heavy metals have maximum permissible level in water as specified by these organizations. Maximum contaminant level (MCL, 2003) is an enforceable standard set at a numerical value with an adequate margin of safety to ensure no adverse effect on human health. It is the highest level of a contaminant that is allowed in a water system. The two elements that were studied in this research namely: Zinc and Silver have Maximum Contaminant Levels of 5mg/L and 0.10 mg/L for drinking purposes, according to National Secondary Drinking Water Regulations (NPDWR, 2009).
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