Rainwater Harvesting Filters Explained

Most people do not think about how they get the water they use; they just turn on the tap and the water is there. The same holds true for rainwater harvesting (RWH) systems. Most people understand there is a cistern that collects and holds the harvested water. But what they may not realize is that with every large scale RWH system there is a mechanical system that pumps, filters, and treats the harvested water before it can be reused.

On-site pumps pressurize the harvested rainwater to carry the water to the end use. Filters are used to remove particulates in the flow path, protect equipment, and improve the quality of the water used. UV lights disinfect the water, and chemical injection provides another layer of disinfection. With all of these components, a certain level of maintenance on the system should be expected to keep the system in good running order. Different system designs require different levels of maintenance, and good maintenance starts with good filters.

Why are filters so important? Filters remove particulate from the flow path. As the particulate removed builds up on the filter, pressure builds as the pump tries to push the water through the occluded filter. The filter will need to be cleaned, or the pressure drop will prevent the system from performing to its intended design flow and pressure. Filter systems are either designed to be manually cleaned or flushed or they are set up with automation to flush them based on the increase in pressure.

Bag or membrane filters (example - whole house cartridge filters) need to be disassembled to remove the old bag or cartridge, and a new bag or cartridge inserted prior to reassembling the filter housing. Systems that need to keep running and cannot be shut down for this maintenance should utilize a parallel filter design or a bypass design to be used for the short amount of time the filter is being maintained.

Screen filters can be flushed to remove the captured particulate. Flushing a filter will allow the system to keep operating without having to disassemble the filter body. The flushed water containing the removed particulates should be plumbed to a drain line or allowed to spill out onto the ground. Some screen filters can be set up with automated flushing routines. This can be an automatic valve opening on the drain line or a complex rotating head inside of the filter body to suck off the material directly from the screen face. Systems with PLC controllers can have programing written that will flush the filter on multiple parameters. Typical parameters include pressure drop across the filter, amount of water pumped from the last flush, and time since the last flush. In addition, automated filters can initiate alarms if the flushing routine does not clear the pressure drop. The alarm would alert that maintenance is required, and the filter body needs to be manually disassembled to clean or replace the screen.

So let’s use that rainwater for more than just a good day to sleep in. With the technology to remove particulate and disinfect, the possibilities for reusing rainwater are endless.