Onsite treatment of Domestic Greywater using constructed Wetland in Ghana
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Date
2021-07-09
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Abstract
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.
Description
A thesis submitted to the Department of Civil Engineering, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfilment of the requirements for the award of Doctor of Philosophy in Environmental Sanitation and Waste Management.
Keywords
Onsite treatment, Domestic Greywater, Constructed Wetland, Ghana