As part of our services we provide clients with design advice on integrated rainwater harvesting systems when they design a building.

Here is an example of a system of a conceptual design review  that a  client has commissioned our team to do.  The house is located  within a coastal estate

Conceptual rainwater harvesting design report

A rainwater harvesting system has been the requirement of the building since conception. A 'green room' has been designed into the residence and is located in the basement level of the building. The size of the room was designed  to house up to 5 standard 5000l vertical round water tanks and other rainwater harvesting equipment. Provision has been made to route the storm water from the roofs into this room. Only rainwater falling on the roof will be harvested. This reduces the risk of contamination to the water inside the tanks.

Conceptual design green room for rainwater harvesting system

 Household water demand

We calculate the  maximum probable flow demand
The household has the following number of plumbing fixtures and appliances:
Water Closets (Toilets): 8
Garden Taps (Estimated): 3
Showers: 7
Sinks/Basins: 9
Baths: 3
Based on SANS 10252-1 fixture method, the maximum probable water demand design flow rate is calculated to be 54.2 litres/min (3252 litres/hr). This is using the most conservative input values.

Household flow and pressure requirements

We calculate the minim required pressure required at maximum probable flow demand
Minimum Pressure required by the plumbing fixtures is 100kPa at full flow rate. This means that the pump must be able to deliver at least 100kPa at the fixture after all pressure losses in the plumbing network have been incurred.
The height difference between the basement where the pump will be situated and the highest plumbing fixture in the top floor is approximately 7.5m This translates to a static pressure loss of 73.6kPa.
The pressure loss across the filter set will be no more than 145kPa for dirty filters at full flow.
The friction loss in the plumbing network should be no more than 150 kPa at full flow rate.
The pump that is specified for the building therefore needs to be able to deliver at least 468.6 kPa (47.76m) at 54.2 l/min.

Estimated daily water consumption

A water consumption of 830 litres per day is estimated. This value is used to simulate the performance of the system.
The projected roof area of the residence is approximately 270m2. Based on experience and virtual system simulations, the optimum rainwater harvesting capacity for KwaZulu-Natal's rainfall patterns is 0.066m*RA, where RA is the roof area.
This means the usual optimum rainwater storage capacity would be approximately 17820 litres for the average rainfall pattern of KwaZulu Natal. The system is being combined with a water back-up system so the water back-up volume must be added to the rainwater storage volume.
The backup volume of the system should not be less than 3 days water consumption. The back up volume is set to 3000 litres for this design. Assuming the RWHS will utilize four 5000l tanks the total water storage capacity would be 20 000 litres, and the with a back-up volume set at 3000l, the RWH volume will be 17000 litres. This is the value that is used to simulate the performance of the system.

We do a rainwater harvesting system simulation

A software package is used to simulate the performance of the system. KZN rainfall data from 1st January 2015 to 31st December 2015 is used to generate the input signal of the system.
The input parameters used in the model are;
1. Roof area: 270 sqrms
2. Storage volume: 17 000 litres
3. Daily water consumption: 830 litres/day,
4. Primary rainwater filter efficiency: 95%.
The outputs of the model are:
1. Rainwater volume in tanks (litres) vs time (days)
2. Overall system efficiency (%)
3. Total rainwater harvested in the period

Figure 1: 2015 Daily Rainfall Data 2015 input signal

 Durban daily rainfall data

Figure 2: Resultant rainwater volume in tanks

Tank volume data simulation


What can be seen from figure 2 is that municipal water will only be consumed when volume is at 0. It can be seen that municipal water will only be consumed during the winter months. The tanks overflow when the volume reaches maximum. In the simulation it can be seen that the tanks overflow twice in the year. This is correct.
The resultant system efficiency is 81%, This means that 81% of the rain that falls on the roof can be expected to be harvested and used in the household. 5% of the water is lost through the filters flush line and the remaining 14% is lost when the tanks overflow. A system that returns an efficiency greater than 80% is good.
A total of total of 179 389 litres of water was harvested in the above simulation. The Household can expect to harvest this much water every year.

Physical Design

The design of the system is such that the most of the system components will be located within the tank room. The only components that will be located outside of the tank room are:
Valve junction to tie into municipal incoming line
Backflow preventer valve
110 transport piping linking the gutters to the tank room
32mm and 25mm HDPE piping linking the system to the valve junction
The proposed system will work as follows:
Rain falling on the roof will be transported to a 160 header pipe with-in the tank room by buried 110 piping tying into the downpipes. The 160 header pipe will feed into a Wisy WFF 150 vortex filter. The filter is self cleaning and requires very little maintenance. It uses on average 5% of the water to flush away the filtered debris. It makes use of a vertical stainless steel mesh screen that screens the rainwater of all particulates larger than 0.28mm.
The filtered water will flow to a calming inlet located within the first tank. The calming inlet sits on the bottom of the tank and prevents turbulent inflowing water from disturbing the bio-layer that forms on the bottom of the tanks.
A skimming overflow located within the first tank will skim all the floating debris when the tanks overflow. The overflow will consist of a P-trap and a vermin screen. This will prevent any vermin from entering the tank. The overflow will be routed to a 160mm drain line that is linked to the flush line of the WFF150.
The tanks will be interconnected in a 'Series' Configuration by 50mm interconnections. This means that the rainwater will flow into the first tank and be drawn out of the last tank. This ensures proper water circulation. Each interconnection will have an isolation valve to enable the servicing of each tank without having to drain all tanks.
A level gauge will be installed on one of the tanks which will always display the level inside the tanks.
Water will be drawn out of the last tank via a floating suction. The floating suction draws the older cleaner water at the top of the tank first and ensures proper water circulation.

Water will be drawn by a pump set mounted inside our standard filtration unit. The pump set consists of a horizontal multistage centrifugal pump, a global water solutions pressure tank and global water solutions pressure controller. The pressure tank ensures that the pump doesn't start in small water demands like rinsing hands and filling a glass of water. This increases the longevity of the pump ad reduces energy consumption of the system.
Water will be pumped through the filtration unit. The filtration unit consists of a 20 micron filter, 1 micron filter, activated carbon and KDF filter and a UV steriliser. The 20 and 1 micron filter remove all suspended particulates larger than 1 microns from the water. The AC&KDF filter treats the water for any odour or colour that might be present in the water, and the UV steriliser kills any microbes that might be present in the water. The water will then be transported to the valve junction at the municipal tie-in point.
A municipal back up will be included in the system. This consists of a float switch inside the tanks that can be raised or lowered to adjust the back-up volume within the tanks, it will be set to provide 3000 litres of back-up water, and a solenoid valve. The municipal water will be routed through the calming inlet as well to ensure that it does not disturb the bio-layer either. The back-up system will be fed by buried 25mm piping that ties into the valve junction at the municipal tie in point.
A reduced pressure backflow preventer will be installed in the municipal incoming line. This prevents the possibility of rainwater from flowing back into the municipal supply. This is a legal requirement and forms

Detail costing report

As a final step we provide the client with a detail cost and installation report.

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