All of us in the water industry take it as read that we need to use water wisely and recycle wherever possible. I put this to the test last year, first at home and then by trying to help my old college. The first attempt at home was straightforward and involved rainwater collection from my roof for garden irrigation. This should surely be a no-brainer. Purchasing a 1 m3 tank and connecting it up with old fittings and reused pipes probably had a payback of three years. So far so good. However, irrigation water is a very small proportion of annual usage in the UK. And as the table above shows, the need for irrigation is irregular and is unlikely to coincide with periods of heavy rainfall! The more important prize is to recycle more challenging water sources.
Colleagues of mine have been more ambitious and extended the scope of reuse for their homes. I was keen to see how the economics worked out for a project with wider scope. However, retrofitting water recycling to an existing building is going to be more economically challenging than incorporating the technology into a new build so it can be hard to justify schemes on economics, or even on environmental footprint terms due to the embedded carbon etc.
I was intrigued to learn that my old college was planning to build seven new residential buildings, each accommodating 20 students, in a significant development in north Oxford. Surely water recycling here would make sense if it was included in the building design early enough? I contacted the college to enquire as to whether they were planning to recycle water? They said that they had no plans to do so, and I offered to have a quick look at the feasibility of a potential scheme.
The first assumption I made would be that there would be an incentive to implement such a scheme from local government, or by the planning authority, or the water provider. Oxford is known as an area where both drinking water supply and wastewater treatment are stressed, especially in the light of major developments proposed for the county. I assumed the City itself would be the first part of the county to get its house in order. However, no such incentives exist.
I looked at options of capturing rainwater, collecting and treating greywater and full-scale treatment of blackwater. A hierarchy of potential uses can then be explored starting with irrigation, then going on to toilet flushing, dishwasher/washing machine feed, and ultimately drinking water. The hierarchy is shown in the table above and illustrates the challenge of matching volume, availability and quality requirements. Clearly, irrigation followed by toilet flushing represent the lowest hanging fruit since these uses are significant but require less treatment. Also testing and quality monitoring are less onerous, and this could be a key factor in determining feasibility. At the other extreme, drinking requirements are a very small proportion of total usage, and it would be hard to justify treating and testing to meet this standard.
I therefore focused my attention on a combination of irrigation and toilet flushing since irrigation is too small a use on its own. Also, when irrigation is required, there is unlikely to be sufficient rainwater unless held in very large tanks, which would necessitate pumping and stabilization during storage.
If toilet flushing is added, there would be a continuous turnover of stored water and the requirements are quite high. Upper floor toilet flushing would however require pumping rather than simply relying on the natural head of water storage tanks, so this would make the system relatively complex and would require maintenance. The lowest cost/lowest tech option that I came up with was irrigation combined with ground floor toilet flushing. Even so, the capital cost still required a greater than 20 year payback, so despite my water credentials, I couldn’t recommend this course of action.
The problem for the college development was the fact that it comprised seven moderately sized buildings rather than one large one. Even though in close proximity, the scheme would need a lot of duplication of collection and treatment systems, although on the plus side, there was an extensive roof to collect the rainwater. Unfortunately, the conclusion I came to is that water recycling in the context of small commercial buildings is hard to justify, even for new builds. Water is relatively cheap while engineering solutions to recycle are quite expensive, and there are few incentives from government or suppliers to break this non-virtuous cycle. Simple rainwater collection schemes at the single home level easily make sense, but will not have much impact on a nation’s water footprint. It’s time for higher levels of intervention by the State or local government to provide incentives to do the right thing for both new builds and retrofits and make water recycling the status quo.
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