WORLD OF FIGURES: Harvesting rain water is too expensive, not worth the trouble

Sunday March 10 2019

A pupil  wades through stagnant rain water on Ganjoni Road in Mombasa on November 16, 2015. Harvesting rain water is too expensive. PHOTO | KEVIN ODIT | NATION MEDIA GROUP

A pupil wades through stagnant rain water on Ganjoni Road in Mombasa on November 16, 2015. Harvesting rain water is too expensive. PHOTO | KEVIN ODIT | NATION MEDIA GROUP 

Last week, we found that the typical house in Nairobi cannot collect enough rain water to meet the needs of the people living in it.

The calculations revealed that a three-bedroomed house, for example, would need a footprint of at least 180 square metres.

In addition, such a house would need a storage tank with a capacity of about 75,000 litres to sustain the occupants during the dry seasons. This comes to about 15,000 litres per person.

The question that follows naturally is: assuming there is enough roof area to collect the water, how much would it cost to install the storage tank?

A quick market survey reveals that the largest tank available has a capacity of 24,000 litres. So, the house would need at least three tanks (72,000 litres in total).

The price of one such tank is between Sh240,000 and Sh300,000. Working with the lower figure, the three tanks would then cost about Sh720,000.

This is quite a lot of money, but how does it compare to the cost of water supplied by the Nairobi Water Company?

I went through my water bills for the past six months, hoping to establish a good average unit price, but instead I stumbled upon a perplexing inconsistency — more about that later. It turns out that if the consumption is 15,000 litres per month, the bill would be about Sh1,200.

So, if the piped water was replaced with rain water tanks at a cost of about Ks720,000, it would take about 600 months (50 years) to recover the investment through savings! This brings out the reason why it is important to supply piped water from a central reservoir — the cost is extremely low compared to building/buying tanks for individual houses.

Let’s go back to the inconsistencies in my water bills: In each of the months of July, August, and October 2018 as well as February 2019, I consumed 21,000L, but I was charged Ks1,818; Ks1,778; Ks1,784; and Ks1,811 respectively. Now, since the amount of water consumed in these four months is exactly the same, I would expect the bills to be identical, but they are not. Furthermore, the amounts billed are not in agreement with the rates published in the water company’s website.

The published rates say that the first six cubic metres are charged a flat rate of Ks204. After that, the rate is Ks53 per unit up to 60 units.

In addition, the cost of sewerage is calculated at 75 per cent of the consumption amount. Finally, there is Ks50 for the meter rent.

Thus for 21 units, the bill should come to Ks1,798.25. None of the amounts charged in the four months is equal to this figure.

Some one suggested that the discrepancy is due to the amounts carried forward from previous months.

This is not so: the figure quoted above are for “Current Bill”. That is, before the carry forward values are added. The Nairobi Water Company needs to move quickly to sort out this discrepancy.
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