A Decade of Stable Two-Stage Biofiltration Performance at Hutchinson, Minnesota

With Eric Levine, City of Hutchinson, MN

In 2006, the City of Hutchinson, Minnesota decided to solve its problems with corrosive groundwater by constructing a new 6.5 MGD water treatment plant using two-stage biofiltration in parallel with membrane softening. In this application, the first stage of biofiltration is used to remove iron and the second stage is used to convert ammonia to nitrate. Manganese is also removed through the two stages. Initial operation of the plant proved promising (Henrie, 2012) (Nagy, 2008).

The biofiltration system has operated stably over the last decade and has contributed to performance improvements in water distribution system, the water reclamation plant and biosolids management. Water quality improvements reduced distribution system corrosion, bringing system copper levels from 2.2 mg/L to below the EPA action level for copper of 1.3 mg/L. Lower distribution copper levels result in lower wastewater influent and treatment plant effluent copper levels, and lower copper levels in biosolids. With the water system in compliance with the Lead and Copper Rule (LCR) requirements, the corrosion inhibitor was reduced over time and is now discontinued.

Stable operation The adjacent figures illustrate the iron, manganese and ammonia reductions. The figures show that removals were stable throughout. To encourage biological removal of iron and manganese, oxygen addition to the first stage of biological filtration was limited using an in-line pressure aeration system using dissolved oxygen probes or ORP meters as shown in the simplified process diagram illustrated in Figure 4. Dissolved oxygen probes proved to have lower maintenance and thereby provide more reliable process control. The second stage nitrifying filter was supplied with additional oxygen through a second stage aerator. The second stage consistently reduced ammonia to below detection limits and has reduced chlorine demand by 15 mg/L.

Corrosion Inhibitor Unnecessary The City of Hutchinson was plagued with high corrosivity of their drinking water supply, contributing to high copper levels in their distribution system. Table 1 displays the source water characteristics. Table 2 displays the LCR compliance data prior to the plant commissioning in 2007. The biofiltration system contributed so well to the corrosivity reduction that the corrosion inhibitors became unnecessary. The original corrosivity of the water had compelled the City to use the maximum dosage of corrosion inhibitor in order to maintain compliance with the requirements of the LCR. High phosphates in the water caused additional costs for large local electronics manufacturers producing ultrapure water onsite. High phosphate addition created a significant phosphate load to the wastewater treatment plant. Table 2 also shows the LCR compliance data following commissioning of the plant in 2007 through 2019. The improved water quality produced by the new water treatment plant enabled the community to comply with LCR requirements. The City slowly reduced corrosion inhibitor addition to zero and maintain LCR compliance. It is estimated that eliminating the corrosion inhibitor reduced the phosphorus load to the wastewater treatment plant by between 250 and 400 pounds per day.

Turndown to 25% The biofiltration system required significant turndown due to low winter water demands and proved to be very resilient. The proportion of biofiltration effluent to permeate could be varied from 40:60 to 25:75. The system could be turned down to 25% of design flow using one membrane softening train and by alternating biofilters daily. Turn up was accomplished by increasing flow of the biofilters gradually (20% per day) and turning on a second and third membrane softening train. Figure 5 shows the historical flow contributions from the biofilter and membrane softening systems.

Operator Intervention Reduced The biofiltration system replaced a 1950’s IDI Greenleaf filter and a General Filter CenTROL system that required operator intervention for backwash. Fully automated backwash systems were tuned at startup to backwash every 70 hours for the first stage and every 120 hours for the second stage. New control and security systems enabled monitoring through the city-wide fiber-optic network and control access at the water treatment plant, at workstations on the City network, or on City mobile devices. This enabled the City to have one operator stationed at the wastewater treatment plant monitor and operate the water treatment plant on weekends. The plant-wide camera system enables fire, police and maintenance staff to monitor chemical deliveries or sensitive operations without being present.

mpact on Water Reclamation and Biosolids Once the water treatment plant was commissioned, distribution system copper levels dropped significantly. Figure 6 displays the water reclamation plant influent and effluent results before and after the commissioning of the water plant – from between 240 and 310 μg/L to an average of 101 μg/L – a reduction of roughly 68%. Prior to the water plant commissioning, copper levels in the resulting biosolids were 1,965 mg/kg – above the Minnesota limits for Class A Biosolids. Table 3 displays the copper content of the biosolids before and after the plant commissioning in 2007. The current copper content of the biosolids is 554 mg/kg, a reduction of 72%.