The city of Hudson, N.Y., had problems meeting effluent water quality treatment goals due to aging slow sand and diatomaceous earth (DE) filter systems. The key problem areas were total organic carbon (TOC), disinfection by-product precursors and turbidity reduction. Turbidity spikes were caused by the DE filters failing periodically, in addition to halo acetic acid and total trihaolomethane exceeding regulatory limits.
Knowing that they needed to improve treatment performance and expand their plant capacity, the city hired Stearns & Wheler, LLC to evaluate a number of treatment technologies. In 2001 they selected the Trident process  , part of the Microfloc Products  offering of Siemens Water Technologies, to be pilot tested.
The Trident process  consists of upflow adsorption clarification followed by mixed media filtration in a packaged treatment system. Prior to entering the adsorption clarifier system  , coagulant and polymer are added for coagulation. Flocculation and solids capture occur in a bed of buoyant clarifier media contained in the clarifier section. The clarified water is then filtered through a mixed media bed consisting of anthracite, sand, and garnet. Each layer of filter media has a progressively smaller effective size and increasing specific surface area to form an ever-tightening filter bed. Both the clarifier and filter are periodically cleaned using an air-water flushing system.
The raw water is supplied to the 26-acre Churchtown Reservoir from Taconic Creek.
From Churchtown Reservoir, the water flows by gravity to the head of the treatment plant located eight miles away. Prior to July 2005, the treatment consisted of either slow sand filters (circa 1910s) located at the reservoir, or DE filters (circa 1960s) found at the water treatment plant in the city. The city primarily used the slow sand filters but had the option of operating the DE filters if necessary.
During the pilot study the source water ranged from 1.8-3.0 NTU turbidity, 4-15.7 color units, and pH of 7.1-8.1. The two-stage adsorption clarification and mixed media filtration provided with the packaged Trident system was found capable of producing exceptional filter effluent quality and good net water production. More specifically, the filter effluent quality met the protocol requirements with turbidities less than 0.1 NTU and filter effluent color of < 5 color units. During testing the clarifier run lengths ranged from four to six hours, while filter run lengths approached 24 hours.
Construction of the 2.8 MGD facility was finished in the spring of 2006 with four stainless steel Trident units installed. The chemical treatment process includes PCH-180 poly-aluminum chloride to coagulate soluble organic materials, and Cytek 1986 and Polyamine polymer as a filter aid. The Trident system is cleaned based on time duration with the adsorption clarifier system flushed every four hours and the filters backwashed every 40 hours of operation. Finished water is pumped to a 2.5 million gallon storage tank from a 350,000 gal. on-site clearwell. Chlorine is added as a disinfection agent prior to the distribution system. The entire system is PLC controlled, including automatic coagulant feed adjustment based on water quality produced. A SCADA system provides up-to-date real-time graphic visual displays and report generation.
The city is conducting monthly TOC measurements and currently meets the required 50% removal consistently with this new water plant.
Further refinements to the pre-chlorination system are in process at this time.
The new drinking water treatment plant services the 7,500 residents and businesses of Hudson, N.Y. The design capacity will also meet the city’s growth needs for the foreseeable future. The total cost of the project was $9.6 million. The Drinking Water State Revolving Fund assisted the community with a low interest loan and funded a portion of the project with a hardship grant.