Characterisation of faecal sludge from urine diversion toilets: impact on black soldier fly larvae growth.

Abstract

The area managed by eThekwini Municipality in KwaZulu-Natal, South Africa, was extended in 2002 to include approximately 80 000 households without basic sanitation. The municipality adopted urine diversion dehydrating toilets (UDDT) as a dry on-site sanitation option. The adoption of the UDDTs over other sanitation methods was due to UDDTs being cost effective, the geographical location of these households and the impracticality of providing a sewer system. The UDDTs offered waterless sanitation in a water-constrained environment and the pedestal separated faeces and urine at source, making it possible to handle the treatment of the two excreta streams separately. These UDDTs overcame the problem faced during the desludging of ventilated improved pit (VIP) toilets as UDDT vaults are smaller and allow for easier manual emptying because of the lower moisture content of the faecal sludge (FS). It was the responsibility of the household to empty the vaults once they were full, bury the FS on the property and plant a tree to mark the burial site. The urine was directed to a soakaway located near the unit. In 2017, the eThekwini Water and Sanitation (EWS) unit implemented an UDDT emptying campaign due to the householders’ dissatisfaction over emptying UDDT sludge and the possibility of exposure to FS sludge after burial on the household property. In addition, there was growing concerns over health risks due to potentially pathogenic sludge. A solution was required to treat the waste from the UDDTs, and through funding from the Bill & Melinda Gates Foundation, a project was initiated to investigate the feasibility of using the black soldier fly (BSF) larvae technology. Under this project, a full-scale BSF larvae processing plant was established in Durban, South Africa, with the aim of treating 20 tons of UDDT FS per day. The BSF larvae have been shown to be consumers of a wide range of decomposing organic matter, including kitchen waste, human waste, animal waste, restaurant waste, and vegetable waste. In 2017, there was very little data available on the characteristics of UDDT FS and the impact of UDDT FS on the bioconversion process using BSF larvae. A study was therefore initiated to determine the bioconversion and waste reduction capabilities using the BSF larvae. This study was undertaken by monitoring and determining different material flows in batch reactors fed on two types of substrates: (i) UDDT sludge, and (ii) a mix of UDDT sludge and primary sludge (PS) by performing mass balances. Due to the unavailability of data from the full-scale plant and problems experienced with commissioning components of the plant, small-scale studies were conducted under the same conditions as at the full-scale plant, and the data from these studies was then used to predict the overall material flows of the treatment plant. The small-scale studies were performed in sheds at the BSF larvae plant under uncontrolled environmental conditions, and data was generated from either on-site measurements, or through laboratory analysis of the substrates before and after treatment. This data was then extrapolated to generate material flows for the full-scale BSF larvae waste management plant. The mass balances were carried out on a dry matter basis, wet matter basis, volatile solids, and ash. The growth of larvae was monitored every three days over a period of 13 days. Laboratory analyses were carried out on the UDDT sludge, and a mix of UDDT sludge and PS to analyse the difference in the composition before and after treatment with the BSF larvae. Laboratory analyses included total solids, volatile solids, ash, chemical oxygen demand, calorific value, carbon to nitrogen ratio, and protein content. The change in depth of the substrates during the small-scale trial was measured to determine the reduction in volume due to treatment with the BSF larvae. Humidity and temperature fluctuations were also monitored in the sheds to investigate the effect of changes in these conditions on the process. Due to poor waste management services in the areas where UDDTs are installed, users of UDDTs generally dispose of other wastes (termed trash) in the toilets, including metal objects. The presence and concentration of heavy metals such as Pb, Cd, As, Cr, Cu and Ni was determined through laboratory analyses of the BSF larvae and remaining substrate (residue) after 13 days of treatment, as the presence of these metals can affect the market potential of the BSF larvae and residue. The outcomes from this project showed that the use of BSF larvae has potential as a process for FS management with resource recovery. Treatment by BSF larvae reduced the mass of UDDT sludge by 28% wet basis in the small-scale trial operating in an uncontrolled and low maintained system and achieved a bioconversion of 6% wet basis. The study also showed that bioconversion could be increased by co-digestion of UDDT sludge with PS as it increased from 6% to 9% with the addition of 15% mass of PS. It was also found that the characteristics of the UDDT sludge directly affected the characteristics of the BSF larvae, which can potentially influence the market value of the BSF larvae. For example, UDDT sludge had a high ash content which resulted in an increase in the ash content of the BSF larvae fed on UDDT sludge. Furthermore, the bioconversion process was shown to be sensitive to temperature and humidity as changes in these conditions affected the relative waste reduction and bioconversion rates. Outcomes from this study indicate that the bioconversion and waste reduction of the UDDT sludge could be improved by co-digestion with an organic substrate and using an environment with controlled temperature and humidity.