Onsite treatment of Domestic Greywater using constructed Wetland in Ghana

Abstract Domestic greywater is the most neglected sanitation component in Ghana, likely due to poor wastewater management in general. Sewerage coverage is less than 10%, and onsite systems mostly cater for blackwater and not greywater. Environmental and public health risks from greywater are inevitable although information on same is scanty. This study aimed at identifying and incorporating indigenous knowledge and practices of greywater disposal into a low-cost green technology like constructed wetlands (CW). First, data collection involved 451 surveyed houses in nine communities. Data was also generated through literature reviews, and laboratory analysis of greywater samples. Horizontal flow subsurface CW was indigenized by incorporating into design local vegetation – taro (Colocasia esculenta) and sugarcane (Saccharum officinarum), and local media (gravels and laterite: d10 = 5.5 mm & d30- 60= 0.1 – 7 mm), and then tested. Eight experimental setups including controls were operated and monitored under residence times (HRT) of 1, 2 & 3 days, repeated for five batch runs (with 187 ml/s feeding for 1hr) between June and October 2018. Findings showed that greywater were disposed of mostly into the open (46–66%), and few (4–24%) by septic tanks/soakaways. Most respondents (84%) perceived plants usage as beneficial treatment agents in greywater disposal. Mostly used plants included sugarcane, banana/plantain, and taro among 36 plant species identified 1,259 times. Greywater characteristics showed high contaminant levels: turbidity (39.4 – 2,880 NTU), BOD5 (64 – 700 mg/L), COD (207 – 2,308 mg/L), TSS (70 – 4,720 mg/L), TDS (420 – 2,860 mg/L), nutrients – TKN, NH3-N, NO3-N, NO2-N (0 – 218.5 mg/L), TP and PO43- (1.24 – 26.18 mg/L), anionic surfactants - AnS (2 – 10 mg-LAS/L), SO42- (13 – 15 mg/L), SAR – 0.6 (meq/l)½, average BOD5/COD ratios ≥0.5, and microbial – TC, FC, and E. coli (2.95 – 10.4 log CFU/100ml). Greywater generation at 95% CI is 39 – 83 l/c/d with specific pollutants loads of 8 – 18 g/c/d (BOD5) and 24 – 48 g/c/d (COD). CW performance showed the following effluent quality (mean ± standard deviation): DO (1.34±0.45 mg/L), TDS (186.5± 30.29 mg/L) and EC (380.17±42.02 µS/cm) all increased and passed discharged limits, but not NH3-N (5.94±1.68 mg/L), P (1.56±1.10 mg/L) and Fe (4.9±3.81 mg/L). SO42- was almost always 100% removed with few exceptions (0 – 2.8 mg/L). NO2-N removal followed SO42- quite closely. Effluent contaminants levels and removal efficiencies also included NO3-N (0.2 – 1.2 mg/L, 81 – 96%) >BOD5 (23 – 37 mg/L, 77 – 90%) >COD (45 – 81 mg/L, 69 – 86%) >TSS (12 – 27 mg/L, 59 – 81%) >AnS (1.3 – 2.1 mg/L, 42 – 75%) and >PO4 (1.8 – 9 mg/L, 30 – 86%). Two-way MANOVA tests showed that effluent quality was significantly influenced by wetlands [Pillai's Trace = 1.790, F(63, 658) = 3.590, p<0.001], and HRT [Pillai's Trace = 0.449, F(18, 178) = 2.859, p<0.001], but not their interactions (p=0.486). CW features like media, vegetation and baffle have influence on performance. Prediction models fitted for main organic contaminants could explain effluent variabilities of 37% (BOD5), 62% (COD), and 73% (AnS). Indigenous greywater disposal practices offer opportunities for low-cost technology adaptation. Ghanaian greywater is polluted, fail wastewater discharge limits, but suitable for biological treatments like CW. The designed CW is effective for treating greywater to acceptable standard by regulatory discharge limits for almost all tested parameters except effluent NH3-N, Fe and P. Yet, further improvement and better understanding of performance of the designed CW under long-term operational conditions are needed.

Source: KNUST