Biofilm Reactors for Greywater Treatment

Abstract

Greywater (GW) is the fraction of wastewater discharged in houses that has not been contaminated with faeces. The average volumes of greywater generated in the households of Bengaluru varies between 40-70 litres/cap/day (50-70% of daily consumption). Greywater treatment and reuse within the houses for secondary purposes such as toilet flushing, gardening, car washing, floor cleaning etc, can meet the shortfalls in water supply especially during the drier periods of the year. It can also obviate the current social barriers and psychological hesitations pertaining to the reuse of treated water originating from large scale sewage treatment plants. The biological methods of GW treatment especially by the anaerobic bacteria can achieve reasonable levels of pollutant removal at comparatively lower operating costs and even a longer duration without any human interference using automation. Any biological method of household GW treatment faces several challenges such as i. lower ratios of organic loading with respect to hydraulic load, ii. inhibitory effects of the synthetic household products, iii interrupted/discontinuous discharge. An average household in urban area like Bengaluru was estimated to discharge 9-14mg synthetic products per litre of greywater. Nearly, 288 different chemical compounds constitute the synthetic household products that enter GW. The growth of anaerobic bacteria in few of such household chemical products especially laundry detergents and floor cleaners showed significant inhibition, On the other hand products such as hair shampoo, toothpaste, hand wash etc were inhibitory only at higher doses. The chemical compounds present in most of the household products such as linear alkyl benzene sulfonate, sodium dodecyl sulfate, triclosan, triclorocarban showed irreversible inhibition of anaerobic bacteria at higher doses. Anaerobic biofilm reactor (ABF) was chosen to treat GW due to its ruggedness, comparatively lower operational energy requirement and low sludge production making it maintenance free for long periods. The long-term performance (>400 operational days) of ABF reactors packed with two different natural fibers (coir and ridge gourd) and synthetic support media (PVC, PE) was studied. The ABF containing natural fibers achieved similar levels of treatment with respect to synthetic supports at lower organic loads of 250-300g/l/d, commonly encountered in households. The presence of chemical compounds in greywater reduced the treatment levels in ABF reactor by 5-6 times with respect to the standard substrates and correspondingly increased HRT. The recycling of GW within the ABF was shown to have a positive impact on the rate of biodegradation. Recycling enhanced the distribution of substrate and metabolic by-products such as volatile fatty acids leading to higher COD removal and 2 increase in pH>7. The use of molecular tools such as Q-PCR and phylogenetic analysis revealed a comparatively lower microbial population and diversity in GW fed reactors, explaining the reasons for lower efficiencies in treatment. The microbiome in GW reactors in addition to typical anaerobes, were dominated by several hydrocarbon and aromatic compound degrading species of α-Proteobacteria, Arthrobacter, Anaerolinaceae, Actinomyces, Synergistaceae. The detection of antibiotic resistance genes (ermC) only in the GW fed reactors indicated the capabilities of microbes to tolerate higher doses of chemical compounds by manipulating their genetic structure. In comparison to the anaerobes, the aerobic bacteria were less susceptible to inhibition by the chemical compounds and achieved 2-3 times higher levels of treatment at similar GW loading rates. A novel macro membrane bioreactor (MMBR) was designed and operated with greywater substrates. The MMBR was a variant of sequential batch reactor (SBR), equipped with a submerged macro pore (50 and 100μ) size nylon mesh. The biofilm formed on the nylon mesh served a dual purpose of particulate filtration and biodegradation of GW compounds. The pore sizes of the mesh and its position within the reactors played a vital role in the COD removal and also determined the rate of fouling and need for simultaneous backflushing. The MMBR attained stability faster than a SBR, however, its long term operations did not result any significant differences in treatment with respect to SBR. The SBR mineralized 470-490mg/l/d COD (85-88%), however 65-75% of the input nitrogen accumulated in the form of ammonia that was attributed to the absence of nitrifying bacteria as revealed by the microbiome analysis. The phylogenetic analysis of aerobic sludge also revealed the presence of polyhydroxy butyrate (PHB) and polyphosphate accumulating microorganism, synonymous of the SBR microflora. The treatment of greywater in a bench scale (100litres feed/day), sequential anaerobic (ABF)-aerobic reactor (SBR) achieved a total COD and nitrogen removal of >85% and 17% respectively, effluent turbidity below 10NTU at a comparatively lower rate of sludge formation in the SBR. A short period of substrate starvation (10days) had no observable impact on the treatment levels in both ABF and SBR. Majority of the AS (LAS) present in GW remained undegraded in the anaerobic stage, whereas, the sequential aerobic phase achieved 75% removal of LAS. This research shows potential to create low sludge, low power requiring, GW recycling options for urban and peri-urban areas.