Physicochemical and Bacterial Community Characterisation of Raw and Anaerobically Digested Faecal Pit Latrine Sludge from Peri-urban Chinhoyi and Potential of the Digestate as Soil Amendment
In an attempt to reach one of the current Sustainable Development Goals for all nations to provide access to adequate and equitable sanitation for all by 2030, many developing countries including Zimbabwe embarked on the construction of pit latrines to improve sanitation in peri-urban settlements. Pit latrines, however, fill up at some stage posing disposal and management challenges both to humans and the environment. This study was conducted to address the current challenges in pit latrine faecal sludge management through: i) physico-chemical characterisation of faecal pit latrine sludge and user habits, ii) anaerobically digesting the pit latrine faecal sludge at mesophilic conditions, and analysing bacterial community structure changes during anaerobic digestion, iii) comparing the pollution indicator parameters in the digestate against the Zimbabwe (2007) and World Health Organisation (2006) regulations for agricultural reuse and, iv) resource recovery from the pit latrine faecal sludge by using the digestate as a soil amendment. Pit latrine sludge samples were purposively sampled from the top layers of six different pit latrines for six times at an interval of 40 days. The sludge was characterised in terms of physico-chemical properties, user habits and management practices before being anaerobically digested in the laboratory at mesophilic conditions at each sampling cycle. Standard laboratory methods were used to assess changes in pollution indicators (BOD, COD and faecal coliforms) during the digestion. Days 0, 14 and 35 were found to be critical turning points of anaerobic digestion. Metagenomic DNA was then extracted from a composite sample from the six pits sludge at days 0, 14 and 35 of digestion and directly sequenced followed by analysis of the bacterial community structure using the Ribosomal Database Project tools. The anaerobically digested sludge was solar-dried and used as soil amendment in greenhouse pot experiments at the rates of 0% (control), 2%, 4% and 6% (w/w pit latrine sludge: soil) and compound C (6% N, 17% P2O5, 11% K2O and 0.1% B) fertilizer. Multivariate statistical analyses were used to determine the variations in sludge physico-chemical characteristics among the pits and determining the most discriminating variables between undigested and digested sludge. The analysis of variance was used to test differences between the greenhouse treatments means. The results showed that the physico-chemical characteristics of sludge from the six pits were significantly different from each other ((R): 0.862 and p < 0.002). The characterisation results indicated that user habits and management practices per each pit were important determinants v of pit sludge characteristics. The main pit contents were the human faecal sludge, anal cleansing material such as maize cobs and kitchen waste. The pit latrine sludge has plant nutrients (N ranging from 0.66 - 2.34% and P ranging from 0.98 - 1.76%) that can be used for crop production. Chemical Oxygen Demand, Total Solids and Volatile Solids were associated with shelled maize cobs and newspapers as anal cleaning materials. Ash addition was associated with pH, K and Mg while P was associated with the addition of kitchen waste. The pit latrine sludge is also highly pollutant (COD ranging from 51 341 - 71 713 mg/L and BOD 14 987 - 22 249 mg/L) and needs to be treated before the resources in the sludge can be harnessed. Total solids and biodegradability index revealed that the sludge can be treated through biological systems such as anaerobic digestion. The process of anaerobic digestion significantly reduced the levels of pollution indicators (p < 0.05) such as COD (42 – 63%), BOD (34 – 52%) and E. coli (from 35 – 60%) respectively. In terms of bacterial community structure, Proteobacteria were the most dominant bacterial phylum in the undigested sludge mixed with 10% cow rumen (24.1%) and were significantly (p < 0.05) reduced to 2.5% at the peak of the digestion (day 14) up until day 35. Firmicutes significantly increased (p < 0.05) from 22.4% to 28.8% at day 14 before being reduced to 11.6% at day 35. The application of solar dried digestate to soil significantly (p < 0.05) increased soil pH, electrical conductivity, plant nutrients (NO3-N and P2O5) concentration. The application of the pit latrine sludge to soil also improved (p < 0.05) fresh and dry weight of the test crop (brown mustard). In this study we proved and conclude that physico-chemical properties can be used to characterise pit latrine sludge. Each pit is different from each other in terms of physico-chemical characteristics. User habits and management practices are determinants of pit latrine sludge characteristics. Although undigested pit latrine sludge has total chemical nutrients which can be used for crop production its use in crop production is limited by high levels of pollutants in the sludge. Anaerobic digestion reduces pollution indicators in the pit latrine sludge and changes the bacterial community structure of the pit latrine faecal sludge composition. Anaerobic digestion promotes the proliferation of phyla Bacteroidetes and Spirochaetes and reduces Proteobacteria and Firmicutes. The digested pit latrine faecal sludge, after solar drying as a post treatment is suitable for use as soil amendment and support crop production.