Weighing the benefits of direct- & batch-style processing systems
Figure 1. Sample Batch Processing-Style Pump Station System Design
With the growing popularity of rainwater harvesting systems, the need for non-potable pump, control and filtration systems of varying capabilities is increasing. Rainwater treatment technology is not as complex as wastewater treatment, but unlike a traditional booster pump for potable water, filtration or treatment is required.
Particle filtration and disinfection are the main treatment technologies seen on modern commercial rainwater harvesting systems. Particle filtration can be automatic screen filters or bag/cartridge-type filters. Common disinfection styles include ultraviolet (UV) light, ozone and chlorine. Designing pump stations that integrate these treatment technologies while balancing cost can be difficult. There are two common methods for pumping rainwater harvesting systems: direct-style systems and batch processing systems.
Direct-Style Pump Stations
Direct-style pump stations pressurize the water needed for the given application—irrigation, toilet flushing, etc.—from discharge of the pump to the point of use. The piping system always is under pressure, much like a household or apartment. When the user calls for water, he or she opens the given valve—flush valve, faucet, irrigation valve, etc.—and the pump station responds in real time.
Since the piping system is pressurized to the point of use, water is immediately available. In this type of system, the filtration component is sized for the maximum flow rate, and if UV is used for disinfection, it also must be sized for that flow rate. Chlorine and ozone are installed on recirculation systems that constantly recirculate water in the tank, and the chlorine or ozone is injected into the recirculation loop.
Figure 2. Irrigation Cycle Profile
Batch Processing-Style Pump Stations
Batch processing systems incorporate a transfer station to pump water at a low flow rate into a day tank, then a delivery station pumps water from the day tank to the point of use. This process uses smaller, less expensive pumps, filtration and treatment on the transfer system, and a normal pump station without inline treatment to pump water from the day tank to the point of use. Ideally, the total cost would be lower than a larger direct-style pump station with filtration and treatment size for the maximum flow rate.
Figure 3. Toilet Flushing Profile
Several factors determine which of these systems would be most applicable to a user’s needs.
Pressure requirements. If the building requires pressure greater than 125 to 150 psi, a batch processing system might be chosen. This would run water through the filtration and treatment devices at a low pressure before it is pumped from the day tank to point of use at a high pressure. Although inline filtration and treatment products are available for purchase in high pressures, the lead time, cost and availability of these components could hinder the overall budget and satisfaction with the system.
Flow requirements. If the pressure requirement discussed is within reason, flow is evaluated to see if a direct-style system or batch processing system is needed. For example, if a toilet flushing system in a commercial building requires 60 gal per minute (gpm) at 50 psi to operate, sizing a transfer pump station to between 20 and 30 gpm would not provide enough cost savings for batch processing to be viable. The difference in size, scope and cost of a 30-gpm pump, filtration and treatment system compared to a 60-gpm system is not a significant enough difference.
Available space for equipment. While irrigation applications can be located indoors or outdoors, plumbing applications are located in mechanical rooms where space often is at a premium. The project may be the ideal application for a batch processing-type system, but due to space constraints, a direct-style pump system with inline filtration and treatment is more effective.
Application or usage profile. When comparing the two most popular applications for rainwater harvesting systems in the U.S.—irrigation and toilet flushing—the water usage profile differs and dictates which system is required. Irrigation systems often operate at or near the maximum design flow rate for hours (see Figure 2). This maximizes the irrigated area within the given water window allowed for irrigation.
Toilet flushing has an unpredictable usage pattern that generally can be described as a low constant demand broken up by short time periods with high flow rates (see Figure 3). Depending on the building, the profile follows the occupancy activity of the building and is affected by the type of plumbing fixtures used.
A direct-style pump station can be used in either situation because it will respond in real time and has the capability to provide the maximum flow and pressure required for the building. Additionally, the filtration and treatment devices on a direct-style pump station are sized for maximum pump station capacity. When applying a batch processing-style pump station to an irrigation application, the transfer pump station is sized for the same flow rate as the delivery pump station, or the day tank size becomes extremely large. These outcomes negate the value of processing water with a small pump and filtration system.
Integrating Treatment Technology
In a batch processing system, filtration and treatment usually are located on the transfer station to save money on components. This means that water already has been treated prior to entering the day tank. Users with batch processing systems can apply another treatment technology on the delivery pump station—residual chlorine or an ozone system—for a second layer of disinfection. The water in the day tank can be recirculated and treated with either treatment technology, too.
Chlorine requires a certain amount of contact time to react with items in water, and thus requires recirculation of the day tank. On a direct-style system, for example, injecting chlorine in real time proportional to flow rate would make it difficult to ensure a residual amount of chlorine in the water at the point of use.
Flow, pressure, footprint and usage profile all have effects on pump station design. Balancing multiple parameters can be challenging, but selecting a manufacturer with experience in the industry for a single source of responsibility will ensure a safe and reliable system for owners.