Controls on the quality of harvested rainwater in residential systems

Abstract:

Rainwater harvesting systems, in which runoff from roofs is collected and used for irrigation, toilets and other purposes, present a viable solution to limited freshwater supplies and excess stormwater runoff. However, a lack of data on the quality of harvested rainwater hinders adoption of rainwater harvesting systems and makes development of rainwater harvesting regulations difficult. We conducted monthly surveys of 6 existing residential rainwater harvesting systems ranging in age from 1 to 11 years measuring pH, temperature, dissolved oxygen, total suspended solids, dissolved organic carbon, and coliform bacteria. We also examined a subset of the samples for iron, lead, mercury and arsenic. Many of the systems routinely met the water quality requirements for non-potable use without additional treatment, which is often required by regulations. In addition, while previous studies have shown that roof runoff contains heavy metals, the water in all systems showed very low or undetectable levels of metal contamination. Coliform bacteria concentration ranged from 20 to greater than 1400 CFU's per 100 mL and correlated with total suspended solids, which ranged from 2 - 7 mg l^-1. The relationship between suspended solids and bacteria population was confirmed in a controlled experiment on the impact of filtering the rainwater before storage. Filtration decreased total suspended solids and total coliforms and increased dissolved oxygen concentration. This project provides insight into the effects of system design and a baseline assessment of the quality of harvested rainwater in existing systems.