Small Scale Decentralized Systems for Greywater Treatment and Recycling

Abstract

As the world's freshwater supply becomes more limited, a greater focus on alternative water resources is required. Wastewater reuse promotes sustainability by lowering global environmental pollution and economic concerns. Greywater reuse and recycling can be essential practices to decrease the demand for clean water. Greywater refers to all domestic wastewater, excluding restroom effluents. Because of the lower levels of contaminants, greywater is easier to treat and recycle than sewage. Thus, greywater reuse is a promising alternative water source that can be used continuously for non- potable purposes. Greywater treatment methods vary depending on site and greywater characteristics. The water quality, quantity to be treated, and reuse applications determine a greywater treatment system design. The present study develops a pilot- scale multi-stage greywater treatment system to treat it to recyclable levels. The study is carried out in two phases. In the first phase, the performance evaluation of a primary treatment unit consisting of an anaerobic-aerobic biological system, followed by a sand filter, was done to remove COD, nutrients, and surfactants. In the second phase, post- treatment of biologically treated greywater by immobilized TiO2 based solar photocatalytic system is evaluated for removing triclosan effectively. In Phase I, the performance of the integrated anaerobic-aerobic-sand filter (pilot plant) is assessed based on the results obtained over 12 months of operation of the system. The removal efficiencies of the pilot plant for COD, BOD, anionic surfactants, TN, TSS, and TP are 89%, 95%, 99%, 85%, 88.5, and 87%, respectively Greywater, predominantly being wash water, where cleansers, mainly composed of surfactants, create huge shock loads and hamper the efficacy of the conventional treatment systems. Therefore, in the present study, experiments were conducted under surfactant shock loads (SSL) to study the reactors’ stability in handling the same. Results revealed that anionic surfactants were removed with efficiencies of 96.02%, 96.21%, 94.81%, and 98.42% for hydraulic retention times (HRT) of 32 h, 24 h, 16 h, and 8 h, respectively. The maximum effluent anionic surfactant concentrations obtained are 44.28, 59.12, 73.35, and 88.36 mg/L under the SSL of 85.94, 121.2, 155, and 180.5 mg /L, respectively. The reactor is recovered to steady-state conditions in about 8, 16, 20, and 28 h after removing the SSL of 85.94, 121.2, 155, and 180.5 mg/L, respectively. A linear relationship with R2= 0.95 indicates that recovery time is proportional to surfactant loading rate increase. Furthermore, the optimum surfactant-loading rate on the integrated system is 19.38 g/m3/h, with a removal efficiency is 91.8%. However, the effluent from the biological treatment unit needs further treatment to eliminate leftover pollutants. Phase II of the study involves developing a novel solar photoreactor. A ternary film of Fe2O3-TiO2/PVP is coated on the glass tube that stands in a parabolic trough concentrator (PTC) for an effective post-treatment of biologically treated greywater effluents. This ternary film of Fe2O3-TiO2/PVP coated on the glass tube is characterized by Field Emission Scanning Electron Microscope (FESEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-visible spectroscopy, and thermogravimetric analysis (TGA). Furthermore, the scratch hardness of photocatalysts at different Fe2O3/TiO2 compositions is investigated based on the width measurement of scratch using FESEM analysis. Results show that at an optimum coating of 5% of Fe2O3/TiO2 composition catalytic film, the maximum scratch hardness (7.984 GPa) is obtained. The photocatalyst has the highest cohesive bond strength and wearing resistance. The degradation of triclosan (TCS) in treated (anaerobic-aerobic treatment system) greywater has been investigated using a solar photocatalytic reactor. Box Behnken design (BBD) has been employed to screen the significant parameters (such as contact time, pH solution, and initial H2O2 concentration) and identify the most relevant interactions between the operating parameters. After carrying out the different trials of the various operational parameters, the response surface analysis has led to the optimal conditions for the yield of TCS degradation, resulting in an 83.27% removal. Based on LC-MS results, it is evident that the photocatalytic degradation of TCS has resulted in eleven intermediate products.