Ready for Reuse

With population growth and shifts in demographics, higher consumption demands on water utilities are putting a strain on water supplies. This has forced regulators and utilities to seek out treatment alternatives to conserve the valuable resource.

As a result, a campaign across the U.S. to initiate a water reuse program has led utilities to review treatment technologies that will enable removal of undesirable constituents and allow reuse for irrigation of parks, golf courses and other nonpotable uses.

Two-Barrier Package Treatment Plant

The Trident HS multibarrier package treatment system was designed to provide two stages of clarification—one stage of media filtration and one stage of ultraviolet (UV) disinfection. The system is better equipped to handle upset conditions and higher solids and hydraulic loading rates. Effluent may be directly reused or treated with reverse osmosis (RO) for further quality improvement.

Water reuse is nothing new, and neither are package treatment systems that incorporate clarification and filtration. In the mid-1970s, the California drought and increased awareness drove the need to find a reliable treatment technology. In the early 1980s, Desert Water Agency set out to evaluate treatment technologies for its secondary effluent from the Palm Springs Treatment Facility (PSTF) in Palm Springs, Calif.

The agency was in favor of the Trident system, as it provided similar effluent quality but was less costly than a conventional clarifier and filter.

Influent to the pilot equipment was taken from PSTF’s existing 10.9-million-gal-per-day secondary plant. The plant consisted of three primary clarifiers, four trickling filters and six secondary clarifiers. The Trident A system consists of two barriers and an upflow buoyant media adsorption clarifier (AC) system followed by a mixed media filter (MMF).

Results from the pilot study showed the package system achieved acceptable effluent quality levels in seasonal variations in summer and winter. Summer influent turbidities varied from 2 to 4 ntu, with the winter turbidities in the range of 8 to 19 ntu. During the winter, the Trident operated at lower chemical dosages and produced less backwash water. This was attributed to the performance of the AC and the control strategy (Aquaritrol process controller) employed during the test run. The Aquaritrol process control automatically regulates the chemical dosage based on effluent turbidity.

The chemical regime consisted of alum and polymer. During the high-influent turbidities (15 to 19 ntu), alum was dosed at a 30 to 40 mg/L range, with a bump of polymer at 2 mg/L. Production efficiencies during the high turbidity phase were 92.4%.

Four-Barrier Package Treatment Plant

Some wastewater treatment plants face in the Washington state area have to remove phosphorus. Pilot studies were conducted at the city of Spokane Water Reclamation Facility in Spokane, Wash. The objective was to investigate tertiary treatment options for additional phosphorous removal prior to discharge to the Spokane River. The primary removal goal was achieving less than or equal to 0.05 mg/L, with less than or equal to 0.01 mg/L as the secondary goal.

The process train consists of treatment by primary clarification, activated sludge, secondary clarification and chlorination/dechlorination before discharge to the Spokane River. Typical secondary effluent quality is 0.5 to 1.5 mg/L phosphorous, with 3 to 8 ntu turbidity, and color is less than 5 color units. Elevated turbidity and phosphorous events are occasionally experienced, primarily due to weather and process variations.

The State of Washington Department of Ecology (WADOE) recently proposed reducing the phosphorous total maximum daily limit (TMDL) discharge limits into the Spokane River to 0.05 mg/L in 2008 and 0.01 mg/L in 2016. This has prompted the city of Spokane to investigate tertiary treatment options for additional phosphorous removal prior to discharge.

The Trident HS system is designed with four barriers: Barrier 1 consists of a tube clarifier section for bulk solids removal with external solids recirculation for improved clarity and chemical reduction; Barrier 2 is an upflow adsorption clarifier that uses patented scarified buoyant media; Barrier 3 is a MMF; and Barrier 4 is UV disinfection. Figure 1 illustrates the Trident HS four-barrier treatment process with the UV disinfection system.

Pilot results showed the system to be a viable technology for the removal of phosphorus (P). During the pilot study, phosphorous in the feedwater averaged 0.84 mg/L ortho P and 1.29 mg/L total P. The pilot unit was able to treat total and ortho phosphorous to very low levels. Filter effluent total P levels averaged 0.015 mg/L, and ortho P levels averaged 0.010 mg/L. Percent removal of total P averaged 98.7% and ranged from 96.6% to 99.5% in the MMF.

The raw water turbidity measured 20 ntu but typically ranged from 3 to 5 ntu. Under steady state operation, the multibarrier clarifier and filter treatment processes achieved the following turbidity performance:

  • Tube effluent turbidity less than or equal to 3 ntu,
  • Clarifier turbidity less than or equal to 1 ntu and 70% removal across clarifier; and
  • Filter turbidity removal to less than or equal to 0.12 ntu.

Results showed the Trident HS is a viable treatment technology for the removal of phosphorus, turbidity and other constituents when properly conditioned. Phosphorus removals of greater than 95% were achieved.