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  • Sarah

Rainwater harvesting

Updated: Mar 16, 2023


Introduction

Water is vital. And when the water table is decreasing, digging a well is not only a risky option for a water supply, but it also actually contributes to the problem of diminishing underground aquifers. The catchment and storage of rainwater may prove a to be a promising alternative.

Rainwater harvesting is as much a science as it is an art. It depends on many environmental factors, but also on the creativity of the captor. In this introduction I will briefly address the considerations when designing a rainwater harvesting system and the motivations for mine.

The two key questions are; for what purpose do you need water and how much does it require? In other words, you should define the usage and the quantity of the water you will need before you start to design your system. If it just for domestic use, then perhaps water tanks will satisfy your water demand. However, if you need water for irrigation, you might want to consider recollecting water using a pond or a cistern (this will be my next project). In my case the usage extents to water for the careful domestic use of two people whole year round.


Secondly, as with all sustainable homestead challenges, you need to observe your environment. What is the expected precipitation per month and per year? Are there any extended periods of drought that are common to your zone? What kind of rains are you likely to experience; all day long drizzle, short showers, rain with a permanent wind direction. Based on this information you start to think about the best collection methods with the space, objects and orientation of your land. I chose to harvest rainwater from my roof for the motivation that it is and steep. The considerable size (almost 100m2) allows to capture a significant amount of rainwater, while the inclination of the roof safeguards it from gathering dirt, and meets some of the horizontal stormy rains head-on.


Finally, you need to decide on a way to store your rainwater. When capturing rain from the roof, your storage will have to be lower than the roof to allow for gravitational flow. There are options for above and underground storage, however a review of their pro’s and con’s is beyond the scope of this post. Personally, I liked the idea of my water being very accessible to be able to clean it and revise its level whenever necessary without the use of (expensive) instruments. Therefore, I chose to have water tanks above the ground.


After this short motivation I will discuss the values, calculations and equipment that I’ve used during the design and implementation of my rainwater system.


Domestic water use

For daily domestic use I calculate:

Activities

Liters

Short shower every other day

20

Dishwashing

20

Handwashing, brushing teeth, cooking and household cleaning

20

Total domestic use

60

Rainwater collection



The surface of the roof is 100m2, the surface of the house is 49m2. I’m not sure how much water it collects in total, but my guess is between 10L and 5L per mm of rain, with drizzle to stormy rains respectively. The gutters are made of zinc and painted with an environmentally friendly protective coating preventing the gutters from rusting. Sanitary tubes are connected to the gutters and lead the water to the storage units, but before entering, the water passes through a large organic matter filter. This filter is rectangular with a diagonal surface that is covered with a maze, filtering out large organic matter such as leaves, that will slide from the inclined filter as they gather.


Before moving on to the water storage I want to leave a brief note on a technique called first flush. First flush prevents the first batch of rainwater to flow into the storage, because it is likely to contain dirt that has been gathering in between the rains. However, it is hard to decide on the optimum amount of rainwater to be disposed of, it requires maintenance and diverters create a weak point in the conveyance system. Therefore, I prefer to keep an eye on the gutters and the roof and divert some of the first rainwater myself throwing a plastic bag over the large organic matter filters. Moreover, I would always advice to check out your harvesting system with the first rain, for example to detect whether there are any leaks. It is then convenient to do the first flush yourself and dispose of the perfect amount of water, without losing too much, neither contaminating your supply.


Then finally, you will want to install an overflow; a tube preventing the tank from overflowing in case the maximum storage capacity is exceeded.



Water storage


For now I have two tanks of 5.400 with a storage capacity of liters to store approximately 10.000 liters of rainwater (the entrance and overflow quit some of the storage capacity). A quantity of water storage necessary considering that this zone is prone to extended periods of drought. 10.000L of storage /60L of daily usage = 167 days or a minimum of 5 months of water supply.



Domestic water supply


Illustration of the set-up of the rainwater harvesting system.


Some things have been left out to keep the illustration simple. The other side of the roof is also equipped with gutters, a large biomass filter and is connected with a sanitary tube underneath the house to the same tank. Connections such as shut-off valves, American valves, etc. aren’t visualized either.


1. Connection between tanks

To sum the capacity to 10.800L.


2. Retention valve


A retention valve keeps the water from flowing back into the tank when the pump is not working. This helps the pump to prime (first water suction), which is necessary when the water level in the tank, and thus the water pressure, is relatively low. A retention valve is efficient when placed at the same height as the lowest possible water level of the tank.


3. Filter for small organic matter


This filter prevents any biomass to damage the water pump. Usually the filter for small matter comes with the water pump itself.


4. Seaflo 33-series diaphragm water pump 24V version


Basically, this water pump provides about 11 liters per minute and is able to retrieve water from up to 1.8 vertical meters (= self-priming distance). It was easy to install the 24 volts version to my 24 volts PV system. Moreover, I have not experienced any failures since I installed it one year ago. I built a box around the pump to prevent the salty air from damaging it.


5. Activated carbon filter


I’m so grateful for this filter to exist! It makes me confident about the safety of the domestic use of the rainwater I collect. Let me explain you why.

Activated carbon filters remove chemicals (such as rust, chlorine and PFAS), gas, bacteria (when the openings are smaller than 0.3 microns) and more contaminants from water. You can think of the carbon as a sponge that absorbs the contaminants. Therefore, it is necessary to replace the filter with a new one every six months.


Filters from left to right; large organic matter, small organic matter and activated carbon.


Conclusion


Rainwater for domestic use might be the most sustainable option out there. However, whether it is compatible with your homestead depends on environmental factors and your personal water use. Nevertheless, I would recommend any domestic use rainwater harvester to install at least the three mentioned filters to reduce the chance of the contamination of your water supply.


For anyone interested in supplying water to an irrigation system I would recommend rainwater collection using a pond or large (underground) cistern, which I will be my next project as well so subscribe to the newsletter for any updates on this subject!


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