Water recycling

 I have just spent the last few days examining a Masters dissertation on rainwater and grey-water harvesting for a student from one of the other South African universities. In the process, I have been reflecting on other projects that I have been involved with over the years, as I am very conscious of living in a water-stressed region, which will become an increasingly important concern in the next few decades. 

First, a clarification on terminology: grey-water refers to any water that has previously been used, whether for bathing, washing clothes or other types of washing, but excludes waste water from toilets or urinals. This is called black water and requires very specialised treatment because of the pathogens in the water. It is technically challenging to purify recycled water, whether from rainwater or domestic use, to meet drinking water standards. For obvious reasons of safety, this is highly regulated. So here I will be focusing on the much simpler consideration of grey-water recycling for uses that do not require human contact, namely toilet flushing and garden watering.

For a number of years we have been recycling rainwater and water from the basins in the building where I have been working at the University of the Witwatersrand. The Civil Engineering building (Hillman Building) dates from the 1940s and has been retrofitted as a pilot project to investigate technical and social aspects of recycling, specifically for flushing toilets. There are several tanks on the roof for rainwater harvesting and a tank in the basement that captures and treats the water from the kitchen and basins to remove any soap and contaminants. Because of the large volume of water needed for flushing in a building that is used mainly for offices and lecture venues, only two of the toilets use grey water. This project has been spearheaded by Prof Adesola "Shola" Ilemobade and has provided a valuable "living lab" for generations of research students who have explored different aspects of grey-water harvesting

An important part of the study was to test people's perceptions about grey-water recycling, with mostly very positive feedback, although disturbing pushback from some of the users, despite the level of education (including one of the lecturers) and the literature that has been posted up in the toilets since the inception of the project. Technically, the project has been very reliable and the water is tested regularly for safety and compliance with the regulations for grey-water.

A couple of years back, we undertook a study for the apartment block where I live to see what our options were in terms of water recycling. We called in a specialist who evaluated the current usage, with specific attention to the estimated volume of water used for toilet flushing. This is the most common use for recycled water, as our consumption of water for flushing is generally about 25% of domestic usage, a huge waste of water of drinking quality which is what we use mostly in South Africa. We also looked at options for using recycled water for our gardens, but here we eventually decided to restore an old borehole that had been out of use for several decades. I am usually quite reticent to advise the use of boreholes, as these deplete the groundwater, but by using this to irrigate the gardens exclusively, the water would mostly percolate back to recharge this valuable resource.

Rainwater tends to be contaminated from the pollutants that collect on roofs which are where we get most of the rainwater that is recycled. These pollutants include particles of dust and more serious contaminants that are airborne, but also include bird droppings. One solution that helps to minimise this in rainwater is to have a system called a "first flush", where the initial runoff from the roof at the beginning of a storm will be diverted from the storage system, carrying away much of the pollution. Even with such a mechanism, some of the pollutants will remain suspended in the water, so the first step in processing is to filter out the larger particles. If the water is to be used for flushing, it can then be treated with chlorine to disinfect and prevent the growth of bacteria. Even if the water is to be used purely for irrigating your garden, bacteria will grow, thriving on the soap and pollutants unless it is used within 24 hours of harvesting.

If water is being harvested from basins, washing machines, baths and showers, an important step is to remove the soap products from the water. Our specialist recommended ozone treatment, although chlorine is a very popular method of minimising bacterial growth. The outcome of the research was that rainwater alone would only serve the building for the rainy season, so to ensure year-round supply, we would need to go for the more complex and expensive option of grey-water harvesting. Here the major expense would be to re-route all the drainage from the baths, showers, basins and kitchen outlets to a storage tank, which would need to be fitted with a pump to serve the five floors of the building.

For single-storey application, grey-water recycling is considerably cheaper and less technically complex, but still requires quite an investment, so it is worth bringing in a specialist who can also model a cost-benefit scenario to see how long it would take to recover this initial expense. As more households look into moving off-grid for their water needs, the price of installation will go down, as we have seen with solar installations, but for now, one can expect a payback period of about 5 years, which was the estimate for my apartment block.