Impact of Greywater Ingress from Cross-connection into Potable Water Reticulation Networks

Abstract

This exercise was undertaken in order to simulate the movement of nitrates and phosphorus within a residential potable water network and to investigate the degree of human exposure to varying quantities of these contaminants from a cross-connection. Greywater was collected from 2 bathtubs and 2 showers, sieved, disinfected, stored and used to flush 2 toilets within a 16 unit residence. The transport of the contaminants was modelled using the kinetics of chlorine in greywater and pathogen inactivation model using the EPANET-MSX program. Results show that the risk of contaminant ingestion is directly proportional to the distance from the point of injection and the degree of human exposure to the contaminants was directly dependent on the demand occurring adjacent to the period of ingress. Thus, if a contaminant is injected prior to or during a peak period, the contaminant is certain to reach all the water use fixtures at a shorter space of time i.e. in minutes or seconds depending on the size of the network. The quantity of contaminants measured at each fixture ranged from 0.12 mg/l to 0.33 mg/l for nitrate and 2.16 mg/l to 9.4 mg/l for phosphorus based on the smallest to the largest injections of nitrate (0.18-0.53 mg/l) and phosphorus (5-15 mg/l). These quantities posed an insignificant health risk according to the South African National Standards on potable water. However, standard precautions must be adhered to in the use of the greywater toilets and maintenance of the GWR system.