Water harvesting

The increased pollution of freshwater sources has resulted in less clean water for domestic and industrial use. Water harvesting helps manage floods and supply water for daily use.  Water harvesting refers to collecting, treating, and storing rainwater or storm water. Storm water harvesting involves accumulating, treating, and storing water collected from creeks, gullies, ephemeral streams, and catchment areas from developed surfaces, e.g., roads, parks, and playgrounds.  Rainwater harvesting involves the collection and storage of precipitation from roofs.

Components of a water harvesting system.

We make use of the following components to collect rainwater effectively.

1. The catchment area – it is used to collect rainwater. It can either be natural, e.g., gulleys, creaks, and streams, or artificial, e.g., rooftops.
2. Conveyance system – transports the collected water from the catchment zones to the recharge zones. We use gutters and drains for rainwater collection.
3. Flush system – flushes out the first spell of rainfall collected. The first spell of water collected contains a lot of contaminants.
4. Filter – for filtering the collected rainwater removing pollutants.
5. Tanks and recharge stations – the primary purpose of recharge stations is to store collected rainwater.

Factors affecting the amount of water harvested.

The amount of rainwater and storm water harvested varies greatly depending on the following factors.

1. The quantity of runoff within a catchment. The greater the runoff, the more the water harvested.
2. Availability of technology – improved technology ensures we can easily collect more water.
3. The capacity of the storage tanks – large tanks enable more water-saving than small tanks.
4. Impact of the environment – areas with a hot climate will evaporate some of the water.
5. Type of roof, slope, and materials – the type of roof and slope determine the amount of water collected. The material used determines the quality of water collected.
6. Frequency, quantity, and quality of rainfall
7. Speed and ease of infiltration through the subsoil – fast infiltration rates mean less runoff hence less water collected. Clay soils are more favorable for collecting runoff than sandy soils.

Types of water harvesting systems

Water harvesting systems come in designs varying from simple to complex structures. The types of rainwater harvesting methods available are;

Water butt

Water butt harvesting systems collect water from natural rainfall or drain pipes. Collected Water is mainly used for watering the plants.

Indirect pumped

This water harvesting system does not rely on gravity to supply water to the outlets. The owner pumps the water collected to the tanks. The use of pumps means you can place the tank at any height. Tank placement at any height enables great flexibility to adjust the flow and pressure of water according to the requirements of a building.

Directly pumped

Directly pumped uses pumps to supply the water collected to the building. The systems can use submersible or suction pumps. Water backup from the main supply must be directed to the underground tank first before being pumped for use. We install the submersible pump in the underground tank, whereas the suction pump is placed within the control unit of the house. Using suction pumps means we don’t need to be direct water from the main supply to the underground tank. The suction pump can deal with the backup and water from the underground tank. Submersible tanks are much more efficient than suction pumps.

Indirect gravity

These water harvesting systems supply water to the outlets by gravity alone. Water collected is first pumped to the header tank, usually located at a high point. Water then flows by gravity to the outlets. The pump only works to deliver the water to the header tank.

Gravity only

These water harvesting systems use gravity only to deliver the water. The tank needs to be located below the gutter system and above the outlets. This system is energy efficient since there is no usage of pumps. It saves on costs.

Retention ponds

We usually use retention ponds to collect runoff. These ponds improve water quality through natural processes .g., sedimentation, solar disinfection, decomposition, and soil filtration. We use concrete and mud to make the base of the retention tanks, and the water is used for watering the livestock, groundwater recharge, irrigation, etc.

In-ground storage

In-ground storage systems make use of underground tanks. Their use is widespread in areas that receive rainfall in a single season. The tanks are insulated, reducing the rate of evaporation, and the water doesn’t freeze if placed below the frost line. These systems make use of electric pumps to deliver water to the outlets.

Techniques of water harvesting

We use the following techniques in water harvesting methods, i.e., rain barrels, dry systems, wet systems, and green roofs.

Rain barrels

This method is common in many households. It is easy and affordable to install, and the tanks can store significant amounts of water. The water tank is placed below the downspout of the gutter systems. The conveyance system directs the water to the water tank. Water collected can be used for domestic use or in drip irrigation systems.

Dry systems

Dry systems are similar to rain barrel systems, only larger tanks are needed, and the gutters divert water to the large tanks. The use of this technique is quick and cheap to implement.

Wet systems

In these water harvesting systems, you place the tank and collection pipes underground. The collection pipes are connected to the downspout of the gutter system and directed to the underground tanks. The tubes need proper maintenance to reduce the chances of leakage.

Green roof

Green roof techniques don’t need storage tanks. The water collected is directed to gardens or farms directly. This technique involves installing drainage systems from the roof to the garden. The maintenance costs of this method are low.

Design criteria for rainwater harvesting

Proper water harvesting designs ensure that the collected water is enough to cater to the required needs. Factors to consider when designing a rainwater harvesting system are;

1. The use of the harvested water – the intended use of the harvested water affects the design of the harvesting system. If you want harvested water for indoor and outdoor use, then more precision is used to maximize on the available rainfall.
2. Reliability of the system – you will need a reliable method if the water demand is high and the supply is low. Reliable systems maximize the available rainfall collecting as much as possible.
3. Size of catchment area needed – the roof size determines how much water will be collected.
4. Location of the catchment area relative to the intended use
5. Required size and type of storage – the volume of water to be stored should exceed the demand.

Determination of water demand.

To determine the storage size and the system to collect rainwater, you should compute the total water demand. If you use rainwater for indoor use, it is easy to calculate the average volume of water used within the house. You can calculate water for irrigation from the plants’ water demand. Plant water demand varies greatly depending on the growing stage, the type of plants, and the soil type of an area. You may need the help of agronomists to help you determine the exact quantities required.

Determination of how much water can be captured

The amount of water to be captured is determined by the size of the catchment area and rainfall received. Not all water collected during storms is conveyed to the storage tanks. Water losses occur through evaporation, splash outs, loss during the first flush, overshoots from gutters in heavy rains, and leaks. The roughness of the collecting surface also affects the quantity of water collected. Roofs with rough surfaces trap rain in their pores, and the water is lost through evaporation. Water harvesting efficiency is also limited by the inability to collect all the water during heavy rainfall. The system loses water as an overflow when the storage tanks are full. For these reasons, we usually use 75% to 90% efficiency when computing the volume of water collected.

When computing the total water capturing surface, we consider the total area of the roof surface. We take into account areas supplied with guttering systems. Also, we use these surfaces because only these areas can collect the water for use. We take the total roof surface regardless of the slope.  To compute the total monthly volume of water collected by a roof, we multiply the monthly average rainfall received in the area by the entire roof surface area.

The total volume of water collected per month = Total roof surface area × average monthly    rainfall received × Efficiency of water collection (ranges from 75% to 90%)

We design the catchment and storage capacity to be able to cater to the demand during the most prolonged interval period without rain.

Effect of different roof materials on the quality of water harvested.

We mostly recommend the use of metal roofs in water harvesting. Water harvested using metal roofs tend to have less fecal matter bacteria concentration than other materials. Concrete tiles and cool roofs also harvest quality water with fewer bacteria. Water harvested from green and shingle roofs tends to have high dissolved organic carbon concentrations. The high organic carbon concentrations will result in high concentrations of disinfection by-products after chlorination.

Storage tanks

Storage tanks are expensive compared to other components of a water harvesting system. The tank’s purpose determines the materials, size, and location. The tank size should be proportional to the monthly water demand, monthly rainfall, and the size of the catchment area. We place tanks either underground or above the ground depending on; the surrounding landscape, underground utilities, costs, aesthetic preferences, and the capability of the foundation soils to support the tank.

Underground tanks need the construction of maintenance holes to allow easy repair and maintenance. We place above-ground tanks on flat surfaces so that the ground can withstand the tank’s weight full of water.  At Eunidrip irrigation systems, we have qualified engineers who can advise you on which concrete mixture to use to support your tank. Above-ground tanks should be opaque to prevent algae growth and degradation because of ultraviolet light. Common materials used to make tanks are plastics, concrete, wood, fiberglass, and galvanized metal. Each material has its advantage over the others.

First-flush diverters and inlet protection

The level of cleanliness of water harvesting depends on the water use. Water for drinking will need to be cleaner than water for washing. Screen filters are usually fitted at gutter inlets to remove large particles like sticks and leaves. First-flush is the first part of rooftop runoff that contains a high concentration of contaminants. The first flush is diverted away from the tank using first-flush diverters. The diverter prevents water from flowing into the tank until its chamber is full. The first-flush diverter should divert 10 liters per 24 meters square of the catchment area.

We use gutter screens and roof-washers together with the first flush diverters to prevent mosquito breeding and sediment build-up in the tank. Roof-washers use large screens that remove large debris, and we place them between the first flush diverter and the tank. We use calming inlets to minimize bottom sediment disturbance where the water enters the tank.

Conveyance system

Gutters, downspouts, and pipes make up the conveyance system.  It is recommended the gutter system should be able to carry the runoff of the 100-year/1-hour rain event. PVC, seamless aluminum, and galvanized steel are materials used to make the gutter and pipe systems. Guidelines to follow when designing the gutter system during water harvesting are;

1. The slope of the gutters should be about 0.5%
2. It would be best if you use expansion joints 18 meters or longer
3. Limit horizontal conveyance pipe bends to 45⁰.
4. Place gutter hangers 0.9 meters in the center.
5. One downspout per 15 meters of the gutter length
6. Cleanouts on horizontal conveyance pipes every 30 meters.
7. Slope of 2% of horizontal conveyance pipes

Overflow pipe

The overflow pipe directs water from a full tank while preventing soil erosion in the region. The pipe is installed at the top part of the tank and designed to handle the same flow as the gutter system for effective functioning. You can direct the excess water to gardens or nearby water sources.

Outlet pipe and distribution system

We recommend installing the outlet pipe at least 15 centimeters from the bottom of the tank. The outlet pipe draws water from the tank to the distribution system. You may need a pump to provide enough pressure to deliver the water. You can use solar pumps, diesel/petrol pumps, or electric pumps.

Maintenance of water harvesting systems

1. Removal of the first flush after every storm.
2. Inspection and repair of screens
3. Remove debris from the roof, gutter screens, roof-washers, leaf guard, and first-flush diverter after water harvesting.
4. Storage tank draining once per year to remove sediments
5. Servicing of the pumps
6. Emptying the rain barrel during winter to prevent freezing
7. Cleaning out the tank using a bleach solution and then rinsing it afterward.