It’s no secret that rainwater harvesting systems are becoming more prevalent for commercial operations. Recently increased regulations on stormwater runoff and added investment to build more sustainable projects are causing rainwater harvesting systems to gain more attention in the construction industry. Tanks Direct continues to be at the forefront of rainwater harvesting system design, supply, installation, and service. Most of these systems are designed to be very efficient and with fairly straightforward mechanical components, however, they will cost the end user problems if not maintained properly. In the upcoming weeks, Tanks Direct will be identifying separate major components of a complete rainwater harvesting system and common issues experienced when systems are not properly maintained. This week’s feature focuses on micro-filtration systems also referred to as pre-filtration for UV disinfection.
Filtration Systems for Your Rainwater Harvesting System
The most common maintenance item, both with the frequency of maintenance and issues, is the micro/pre-filter that is used in conjunction with an ultraviolet disinfection system. It is important to understand the pros and cons of each filter type so you can minimize maintenance issues and increase the time the system is working at peak performance levels. Tanks Direct can work with you to determine what filter is best suited for your application. Below, we feature the three most common filtration options found in rainwater harvesting treatment skids. While all of these systems are quite good at filtering water, some are better than others for this application.
Cartridge Filters/Bag Filters
Cartridge filters and bag filters are made of felt/mesh materials that filter particulate out of the water with fine-tuned precision. These filters tend to be on the lower end of the cost scale, but not necessarily the best option if your system is regularly treating large amounts of dirty rainwater. Depending on the cleanliness of the incoming water, these filters can become clogged in very short order (days). This means that the filter is doing a good job, but this also presents a maintenance headache for the end user. When clogged, most systems will present a high-pressure alarm, or something similar. At this point, the filter needs to be manually changed and cannot be reused. Depending on the system being used, a high-pressure alarm could put the entire system into bypass. When in bypass, you are not treating and reusing rainwater! Tanks Direct has witnessed situations where bag filters are being changed on a weekly basis for months after the startup of a system. This means that multiple replacement filter cartridges/bags must be on hand and the system needs to be closely monitored on a very regular basis. While these filters have been used extensively for whole house well water treatment, pool treatment, etc., we are not convinced that they are the ideal type for rainwater harvesting systems.
Auto Backflush (Screen) Filter
An auto backflush (screen) filter is another common solution to meet rainwater filtration needs. These filters typically fall in the middle of the cost scale. Auto backflush filters are designed to filter water by way of a physical barrier (similar to a bag or cartridge filter) but with this type, in most cases, the physical barrier is a series of metal screens. Once the system begins to clog with filtered particulates, the pressure will rise. The system contains a built in pressure sensor that automatically puts the system into backflush when the high pressure threshold is surpassed. The typical backflush operation works when the water in the filter is then reversed, or a pressure differential is allowed to occur. When this happens, a waste valve is opened and any particulates captured are scrubbed from the filter screens and sent down the waste line. Generally, these filters are very efficient and work seamlessly without much outside input. Typically, yearly or so, these units need to be thoroughly cleaned and maintained. The only tradeoff for these systems is that they tend to get quite large for higher flow rates. Another option would be to split the flow and have multiple filters for a given high flow rate system.
The media filter is the most complex filter we see. It is also on the higher end of the cost scale. These filters generally consist of a large fiberglass container with a backflush head that is filled with various types of media. Common media materials include sand, activated carbon, silica, gravel, etc. Media allows for the collected rainwater to be filtered by different types of material. Generally, these systems are quite efficient, long lasting, and in most cases provide the highest levels of targeted filtration and treatment. These filters usually have auto or manual backflush heads which work to scrub the media on a regular basis (weekly or biweekly). Eventually, the media will be spent and the system needs to be replaced. In many cases, the container is vacuumed, and the media is replaced in the field. Sometimes, the entire container is simply swapped out for a new one with media in place. While these systems tend to be maintenance friendly, they do have a high initial capital cost and a high replacement cost.
What happens when something goes wrong?
Maintaining a rainwater filter is so important because if it is not properly addressed the system can be in alarm, shut down completely, or will likely find itself in bypass mode. Sending a rainwater system into bypass mode defeats the purpose. If your system shuts down you’re not reusing rainwater. This can be especially troublesome when your project is under some regulatory requirement to capture, treat, and reuse rainwater. If this is the case, and your system is in bypass mode for extended periods of time, you can potentially be fined. Neglect of rainwater systems is normally where Tanks Direct sees high maintenance and replacement costs.