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It is wonderful to see the citizens of Cape Town begin to embrace the idea that every drop counts in responding to the critical water shortages.

While the sentiment of saving every drop is valid, the process for supplying water for a city of four million residents requires the effort to be justified by impact and cost.

For this guide, we need to determine the most critical factors in evaluating the various options.

Four million residents in Cape Town

There are about four million residents that require access to water every day. The water comes from dams that supply water to the City.

Six dams provide the majority of the water, and collectively they can store 898 221 million litres of water. That is an enormous number; using the maximum storage and assuming we could use all of it, it would provide 615 litres per person per day for a year before running out. That is 12 times the current 50 litres per day residents have been asked to use.

Every water saving, harvesting or creation project looks at what volume can be produced and at what cost.

If it is only a modest amount of water, the effort to distribute it to such a large group of residents limits the cost-effectiveness of the plan.

898 221 million litres of total dam storage

The reservoirs on Table Mountain are often full, thanks to Table Mountain’s effect in forcing warm South Easterly winds up its sides cooling as it does and forming the famous tablecloth.

Rain and condensation keep the mountain and the areas around it moist for most of the year. There are also other reservoirs and water storage locations around the City. Together the five on the mountain and three elsewhere store a maximum capacity of 4 377 million litres of water.

Problem solved, we merely use the steady supply from these water sources.

Unfortunately, if the four million people using the City’s new target of 450 million litres per day, the City would drain the entire supply in 10 days. The City does not rely on it directly, but it does supply many projects and some residents reducing the demand on the main supply.

Now consider the more marginal options that people suggest, with good intention, but without considering the volume produced versus the number of people it is required to supply.

450 million litres target daily total consumption

It does not mean each of us should not do what we can to limit our consumption, but it should allow you to consider potential solutions with a bit more context.

One suggestion is that we harvest water from air conditioners. Fair enough, although they typically do not produce large volumes of water, perhaps 30 to 60 litres per day for a large unit.

That is not a small amount, but are there enough air conditioning units in operation to make a significant contribution?

Using the commercial office as a proxy for principal air conditioning use, property24 reported 115 000 sqm in the CBD in 2013. Assuming the CBD was 25% of the total, there would be about 460 000 sqm of commercial space in the City. Let's assume it has increased by 10% in the last five years. So 464 600 sqm. Assuming all are using an efficient system that would produce about 55 litres per day, then you would need about 8 500 units rated at 30 000 BTU. Those units would typically provide a total of 467 500 litres per day.

That is a considerable amount of water on its own, but when shared among residents, it would only be about 100 ml per person per day.

City bylaws require commercial and industrial air conditioning to have the water condensate feed into the sewage system. So while it may appear that a useful source of water is being overlooked, the water is being collected.

The City plans to recycle sewage water which is a more productive way to use a sufficient volume of water collected in central areas to make processing more practical and delivering it in adequate quantities and a reasonable cost.

Target of 50 litres or less consumed daily

The purpose of this example is not to dismiss it as a suggestion, but rather to illustrate a better process for determining how useful a water source might be.

Doing so might save you from frustration when next someone shares what appears to be a practical suggestion that when adequately considered is not.

What we can say is that when it comes to reducing your consumption and reusing more of the water you have, you are the expert.

For bulk solutions, unless communicated by water department officials, it probably is not best to second guess whether they have considered or are ignoring an apparent good source of water.

This article first appeared on CapeTalk : A simple guide to determine the value of a water saving or water creation idea

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