In January, the 8819-m3/d (2.33-mgd) City of Fruita (Colo.) Water Reclamation Facility celebrated its first birthday.
Fruita is a biological nutrient removal facility consisting of preliminary treatment, two oxidation ditches, two secondary clarifiers, and a second-generation autothermal thermophilic aerobic digestion (ATAD) process that produces Class A biosolids.
With the current ATAD temperatures of approximately 57°C to 61°C (134°F to 141°F), the facility can produce between 28,600 and 53,000 kg (63,000 and 117,000 lb) of biosolids every 3.5 to 12 hours, depending on temperature.
Most of the facility is controlled by a supervisory control and data acquisition system that enables operators to monitor and adjust dissolved oxygen or oxidation–reduction potential in the facility’s oxidation ditches to achieve complete nitrification and denitrification.
The facility’s design allows for its operators to make the slightest adjustments in aeration and create the proper environment to remove ammonia to 0.5 mg/L or less daily, and to consistently achieve 99% removal of carbonaceous biochemical oxygen demand and total suspended solids.
Preliminary and secondary treatment
Preliminary treatment consists of a 3-mm fine screen, which removes all inorganics, including the majority of grit. A grit classifier follows to catch what little grit remains — Fruita’s fine screens are so effective that fewer than six 19-L (5-gal) buckets of grit come from the classifier per week. Then, the influent is pumped to anaerobic selector basins to enhance phosphorus removal.
Next, the flow enters one of the facility’s two oxidation ditches. Each of these 5.5-m-deep (18-ft-deep), 5300-m3 (1.4-million-gal) ditches has four aeration zones. Fine-bubble diffusers enable operators to create an aerobic zone and an anoxic zone to achieve complete nitrification and denitrification.
After secondary clarification, the water enters the ultraviolet disinfection process. This process is designed with seven banks of ultraviolet bulbs — six currently are being used — with the target of reaching less than 50 colonies of Escherichia coli per 100 mL of effluent. In practice, Fruita reaches levels as low as 12 colonies per 100 mL with only a few seconds of contact time. The disinfected effluent then flows into the Colorado River.
Solids from the oxidation ditch settle and are wasted from the secondary clarifier to a waste activated sludge holding tank. These solids are thickened by the facility’s two rotary-drum thickeners to between 7% and 8% solids. The thickened solids age for several days in the ATAD process and then are sent to the storage nitrification–denitrification reactor, which cools the biosolids from about 59°C (138°F) to about 35°C (95°F) and supports the growth of nitrifying and denitrifying bacteria.
The Class A product then is dewatered to between 22% and 30% solids using a high-solids centrifuge. These dried solids will be used to fertilize Fruita’s city parks.
The temperature in the Fruita facility during the last week of January 2012 averaged 7°C (44°F). This also was the week the facility was brought on-line.
The facility is not enclosed, and the oxidation ditches are open to the environment. With the use of six loads of seed sludge from a neighboring facility, the plant’s three operators were able to slowly introduce wastewater flows, increasing the flows daily.
With the facility’s ability to regulate aeration, the operators could monitor the treatment system to ensure nitrification and denitrification, and subsequently were able to meet all discharge requirements within 6 days of the startup. The facility’s design engineers had estimated that reaching this level of treatment would take 30 to 45 days.