Disinfection of Unequalized SBR Effluent Using Non-Contact UV

Grundy Center, IA operates a two-tank sequencing batch reactor (SBR) system. Their renewed permit, issued February 1, 2014, included a new requirement for disinfection. Grundy Center hired FOX Engineering to provide engineering and construction services for their disinfection project.

The plant has a permitted average wet weather flow of 1.2 MGD and maximum wet weather flow of 3.0 MGD. The SBR uses a floating weir style decanter, which allows for a variable discharge flow. At top water level, the unit allows 2800 gpm, while at bottom water level the flow is about 1400 gpm. The system has a storm mode which allows for both tanks decant at the same time; this means the maximum flow rate from the system could be as high as 5600 gpm.

The existing system had no effluent equalization. FOX evaluated whether to design for this larger flow or install a new intermediate tank after the SBR system. The tank would allow for the maximum wet weather flow, reducing the size and therefore the cost of the unit. It would also allow the system to remain operational all the time; without equalization, the system would need to turn off multiple times per day. After a cost/benefit analysis, the larger system without equalization was selected.

UV disinfection systems use ultraviolet light (created by applying a voltage to mercury vapor) encased in a bulb to penetrate the cell wall to alter the DNA/RNA of the organism to prevent replication, thus providing disinfection.

The advantages over using chlorine for disinfection? The most obvious is that UV disinfection does not require use or storage of hazardous chemicals. With chlorine, any residual will need to be removed to protect the receiving body. UV disinfection doesn’t have that issue. UV disinfection is not affected by changes in pH, requires a shorter contact time, and has no issue with overdosing. Overall, it is easier to install and operate than a chlorination/dichlorination system.

And the disadvantages? The equipment cost for UV disinfection is much higher than chemical feed systems used for standard chlorination/dichlorination. It does not provide effective virus removal at typical doses, which is an issue for drinking water applications, but not an issue under the current permit requirements. Some wastewater types may have a low transmittance to the light; this increases the power required for adequate inactivation, which increases its already high equipment cost.

Iowa DNR has several design requirements for UV disinfection systems. The minimum design dose for the system is 30 mJ/cm3. This design dose may be higher based on the transmittance of UV rays in the wastewater, a factor called UVT. Collimated beam tests should be used to determine the actual dose required for specific wastewaters. As with other systems in wastewater, the unit requires redundancy. In this case, the system needs to treat 50% of the peak flow with one unit off-line. The system needs to be designed for cleaning without disassembly. Finally, a mechanism designed to prevent flow gradients is required to prevent short circuiting. This is typically an effluent weir to prevent flow variation vertically and possibly a flow straightener to prevent horizontal flow variations.

Due to unique system issues at the site, design requirements for Grundy Center included first, the system shut-off between decants, which is typically 10 times per day. Since the system is designed for the higher decant flow, turning the unit off between decant cycles saves significantly on power costs. The system needed to fit the existing hydraulic profile, even during the high double decant flow. The permit only requires seasonal disinfection, so the unit needs to be amenable to decommissioning over the winter.

FOX Engineering evaluated several types of UV disinfection equipment. Traditional systems use a lamp filled with mercury amalgam and an inert gas surrounded by a quartz sleeve which is then immersed in the water to be treated. The surrounding water keeps the bulbs cool. Non-contact systems have the water flowing in a plastic tube surrounded by the bulbs. Since they never touch the wastewater, quartz sleeves are not necessary.

To save power, traditional UV disinfection systems dim as flow changes. Non-contact units use hydraulics to control dose. As the flow to the system increases, the water level in front of the tube wall rise, filling more tubes. An ultrasonic level transmitter sends a signal to the control system, which then turns on the appropriate number of bulbs.

Figure 1 – Traditional UV Disinfection System

Contact UV systems use amalgam based low pressure, high power bulbs. These units are sensitive to frequent on/off cycles, which can lead to premature burn out. The lamps in the non-contact units use non-amalgam low pressure, high power lamps. These are designed for up to 24 starts per hour. They are also a lower cost lamp, since they aren’t required to dim and don’t need the quartz sleeves.

Figure 2 – Non contact UV Disinfection System

Non-contact units come in two basic configurations, channel mounted and self-contained. Channel mounted units would be grouted into place, using the existing tank walls. The units are accessible from the top for general maintenance and repair. Self-contained units come with stainless steel walls and connect to pipes. Grundy Center decided to use the self-contained configuration, as the heat exchanger coils would be accessible from the side as well as the top.

The new UV disinfection system at Grundy Center is a single unit with two banks; each is designed to treat the maximum single tank decant flow. During a double decant, both banks will turn on. This configuration allows for 100% redundancy during normal operation, and 50% treatment at the design peak flow, meeting IDNR regulation.

The flow proportional dosing allows for automatic control of the system without the need for additional flow meters and control. This feature will save money for the city as the decant flow is normally below the maximum rate. In addition, the design of the lamps allows them to turn off between decants without reducing the life of the bulbs.

This design did raise one regulatory issue. The flow proportioning system allows the unit to have a flow gradient. The bottom rows of tubes may have more head available than the upper rows, which means that the system would have higher flow in the bottom rows of tubes than the upper rows. IDNR regulations require no flow gradients in the system. FOX Engineering worked with the supplier on the design so that when the system had a flow gradient, the minimum dose would always be the 30 mJ/cm3, in accordance with the regulation. During peak flow events, the system would have no gradients. During more typical lower flow events, the maximum tube flow is low enough to provide sufficient dose. Since overdosing the water has no adverse effect, the system would operate effectively without wasting power.

The system was installed and winterized on December 29, 2016. Startup occurred on March 9, 2017 to ready the unit for disinfection season. The city completed their first disinfection testing in May 2017 with the initial results showing E. Coli counts of 10 to 50 organisms per 100 mL of effluent, well below the permit level of 521 #/100 mL.

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.