The City of Poughkeepsie, in New York State, USA, has always been on the cutting edge of water treatment technology. Its first treatment plant, built in 1872, was the first successful slow sand filtration plant in America and the city has continued this tradition of technical innovation with its recent improvement project, begun three years ago. Serving a community of nearly 80,000 people, Poughkeepsies' water treatment facility draws water from the Hudson River and currently meets an average daily demand of 10.5 mgd, with a capacity to treat 19.3 mgd.
The primary goal of the improvement project was to upgrade the existing water treatment plant, built in 1962, by providing new or renovated treatment facilities that comply with current and future drinking water regulations. The plant owners also aimed to increase the treatment capacity from 16.0 to 19.3 mgd. The project included upgrades and modifications to several facilities, including the chemical feed systems, the filtration and disinfection systems, the solids contact clarifier mechanisms, the solids handling system and the high lift pumps.
Disinfection by-products
Another main objective of the project was the reduction of disinfection by-products. To meet disinfection requirements, the facility historically added free chlorine to the process flow stream prior to sedimentation basins. However, this practice posed operational challenges and made it difficult to maintain a consistent chlorine residual. It also required substantial chemical usage and produced significant disinfection by-products.
To address this challenge, a new multiple barrier disinfection strategy was implemented. Primary disinfection is now accomplished using a combination of free chlorine and ultra-violet (UV) light, with secondary (distribution system) disinfection accomplished using chloramines.
The UV installation comprises six Aquionics closed chamber, medium pressure UV units, which provide high efficiency disinfection with online transmittance monitoring. “We get approximately 5,000 hours of lamp life per UV system, but a few lamps have run for as long as 11,000 hours, which is excellent,” commented Paul Lill, the facility's plant manager. “This means we only have to change the lamps about once a year while also reducing our running costs–always an important consideration.”
The addition of UV disinfection facilities downstream of the filters significantly enhanced the plant's disinfection capacity by increasing its effectiveness against a wider variety of microbes of concern. “UV gives us extra peace of mind, knowing that we have an extra barrier in the unlikely event of a filter breakthrough,” said Lill. Since installing UV, the amount of chlorine required for primary disinfection has also been reduced, with a corresponding lowering of by-products by up to 20%. The resulting disinfection levels complied with the new guidelines of the Surface Water Treatment Rule issued by the US Environmental Protection Agency (EPA).
The Aquionics UV systems are situated downstream of the filters and operate in parallel. Their medium pressure, closed channel design disinfects with far fewer lamps and with a much smaller footprint than comparable low pressure systems. Each chamber is fitted with UV monitors to measure actual UV fluence and dose for record keeping. With the addition of an optional online transmittance monitor, real time transmittance values are used to automatically adjust the dose pacing of the UV system.
“We originally considered alternative disinfection technologies to meet our goals,” explained Lill. “The UV units' lower relative cost, their compact size which fits into our existing facility and the technical merit were all deciding factors.”
To keep maintenance low, the UV systems are equipped with automatic cleaning mechanisms which keep the UV lamp sleeves free of organic deposits. Replacing, the systems is a relatively simple operation that can be carried out by on-site staff.
New backwash system
The project also involved the complete renovation of the filters, including the installation of new filter underdrains, filter media, and backwash troughs. In addition, a new combined air-water backwash system was provided to replace the less than optimal water backwash and surface wash that was being used. The filter effluent piping was modified to provide automated rate of flow control capability, and filter-to-waste provisions also added. New electric actuated control valves have been installed, and the various filtration modes of operation were completely automated.
New chemical storage and feed facilities were also built to increase the reliability of the existing treatment processes. The existing chlorine gas facilities were replaced with new liquid sodium hypochlorite storage and feed facilities to improve plant safety. Existing dry lime feed facilities were replaced with liquid sodium hydroxide facilities to improve finished water pH control and make the system easier to operate. In addition, all chemical storage and feed facilities were consolidated into a new chemical building to allow simplified control from a centralized, modern chemical facility.
The solids handling system was also modified to separate the treatment of the two primary waste flow streams: settled solids from the clarification processes (solids contact clarifiers and sedimentation basins), and the waste backwash water from the filters. New sludge pumps and a sludge thickener were added to treat the settled solids. This revised strategy helps improve overall system performance and reliability and also increases the total on-site solids storage capacity.