Designing and Construction of Efficient Trickling Biofilter Systems for Wastewater Treatment

Abstract

Attached growth processes for wastewater treatment have been significantly improved during recent years. Their application can be extended to sustainable municipal wastewater treatment in remote locations and in developing countries for the purpose of organic matter (BOD) removal and pathogenic decontamination. The formation of specific biofilm on support media is the essential part of attached growth processes, having peculiar mechanisms of pollutants removal. The present research work aimed to monitor the successive biofilm development and its physiological activities on polystyrene, tyre derived rubber, polypropylene and stone media, under aerobic and anaerobic conditions. These filter media were artificially colonized with biofilm by incubating it with activated sludge at 30°C±2 for nine weeks. Biofilm formation was monitored by gravimetric weight analysis, spectrophometric absorbance technique, heterotrophic plate count and scanning electron microscopy. The wet weight of polystyrene media biofilm was significantly increased from 1st till 9th week of incubation (0.56 to 1.59 g under aerobic condition). While, in case of other tested media weight of the biofilms increased till 7th week during succession and then started reduction. Relatively less growth was recorded under anaerobic condition as compared to aerobic conditions. Selected pathogenic indicators (Escherichia coli, Salmonella typhimurium, Shigella dysenteriae, Pseudomonas aeruginosa) monitored by HPC/mL considerably declined (90-99%) in the biofilms of all the media under both conditions, signifying microbial reallocation from pathogenic to beneficial microbial community. The MPN index of fecal coliforms and E. coli in the sludge also showed considerable reduction. Correspondingly the decreasing levels of COD and BOD5 (69.9─74.9%) showed signs of sludge digestion by biofilms on selected media types under both the environments. Further, changes in pH and nutrients (nitrites, nitrates, phosphates and sulphates) indicated the other key-organisms with efficient nutrient consuming capabilities in the biofilms. The surface analysis of media by SEM revealed emergence of profound bacterial growth on all media and emergence of cracks on tyre derived rubber media surface and slight deformation was further confirmed by FTIR. Further the microbial community composition in the biofilms of different packing materials was investigated using high-throughput pyrosequencing technology. Sequences of 16S rRNA gene fragments were recovered from biofilm samples of 12 laboratory scale reactors operated at different temperatures i.e., 10, 20 and 30°C for two weeks. Analysis of pyrosequencing and water physico-chemical data showed that substrate type (media vs. biofilm) and temperature conditions influence bacterial community structure and composition in experimental reactors. Greater bacterial diversity was observed in each sample (3142 operational taxonomic units), primarily due to the large number (22029) of sequences available for analysis and the identification of rare species.The number of classified sequences per sample ranged from 1016 to 2919.The results showed that there were 12 phyla and the relative abundance of phylum Proteobacteria was highest (54.06%) followed by Bacteroidetes (28.97%), Firmicutes (5.30%), Actinobacteria (3.88%) etc. in all the samples. The data set illustrated 23 genera of bacterial populations to be commonly shared by all samples, including, Rheinheimera, Rhodococcus, Aquabacterium, Trichococcus, Acidovorax, Flavobacterium, Roseateles, Aeromonas, Sediminibacterium, Hydrogenophaga, Aquimonas, etc., indicating core microbial community in the microbial populations of reactors. In the next step, study was carried out to assess selected packing media for locally designed and lab scale trickling biofilters systems and to develop a simplified model for describing the capacity of BOD removal in trickling biofilter systems. Trickling biofilters with four different media were investigated at two temperature ranges of 5-15°C and 25-35°C. The average removal of both COD and BOD5 was higher than 80% and 90% at temperature ranges of 5-15 and 25-35°C respectively. The geometric mean of fecal coliforms reduction was achieved up to4.0 log10 with polypropylene media at low temperature range of 5-15°C. While at higher temperatures range of 25-35°C reduction up to 3.97 log10 was observed with polystyrene media. A simplified model was developed and used to estimate the optimal BOD loading rates (Bvd) for designing robust trickling biofilter systems, with appropriate filter media which can be capable of treating organic loading rates of higher than 3kg BOD/m3.day. Finally, a simple, robust and a low-cost pilot scale stone media trickling biofilter system for municipal wastewater treatment was establishment at Quaid-i-Azam University, campus. The wastewater treatment efficiency of this locally designed prototype pilot scale stone media trickling biofilter was tested at 20-40.5°C for the removal of different pollution indicators (COD, BOD5, NH4-N, and pathogens). Simultaneously, the biofilms were sampled from the top and deeper layers of stone bed of the reactor for characterization. The Nitrosonoma sp. and Nitrobacter sp. were identified in the deeper layers while, 13 bacterial strains viz., Escherichia coli, Salmonella typhimurium, Shigella dysenteriae, Pseudomonas aeruginosa, Enterobacter aerogenes, Proteus vulgaris, Klebsiella pneumonia, Bacillus subtilis, Staphylococcus aureus, Micrococcus luteus, Staphylococcus epidermitus, Streptococcus lactis and Corynebacterium xerosis were identified in the top layer of the stone media bed. The results signify the COD and BOD elimination efficiency from wastewater considerably increased with passage of time from Day 1 to day 40 of operation (62.4- 98.1%; COD and 56.4 - 98.6% BOD) at flow rate of 1.2 L/min and average BOD5 loading rate of 0.063 kg BOD/m3.day.The average NH4-N levels of the influent were low (0.0024 kg NH4-N/m3.day). However, the result indicated an excellent correlation of the average consumption of 7.55 mg/L of alkalinity per mg of NH4-N removal during 40 days of reactor operation. Moreover, a significant connection between nitrification efficiency and decrease in the average pH range (7.52 to 6.62) was observed, indicating the process of nitrification. The removal of pathogenic indicators from wastewater was evaluated and an average reduction of 88.8% in the MPN index of fecal coliforms in the effluent was recorded. Overall, a significant correlation of COD, BOD5, NH4 ̄N, and pathogenic indicators removal efficiency were noticed with increase in seasonal temperature from 20 to 40.5°C.The overall results proved that pilot scale trickling biofilter has a great potential to be transferred to field scale for treating sewage for small communities in developing countries even at low temperature conditions. It will not only help to improve the public health in terms of removal of wastes and pathogens from wastewater but also treated water could be used for agriculture purposes without any hesitation.