Off the Snails

A pilot study shows that an automated spray system has strong promise for controlling biological growth in UV disinfection chambers

In Connecticut, with the changeover from chlorination to UV for disinfection at some of the water pollution control facilities, operators have seen an increase in attached algal growth and dense populations of tiny pouch snails in the UV chambers.

The result appears to be an artificial increase in suspended solids in the final effluent. Some of these facilities that recycle effluent for use in the plant find that they need to filter it to avoid maintenance problems.

To address the issue, the state Department of Environmental Protection and conducted a pilot study with the Jewett City Wastewater Treatment Plant. The study found that an automated washing system was effective in keeping UV chambers clear of unwanted biological growth.

Feeding on algae

The Jewett City plant, built in 2005, treats 1.1 mgd of wastewater. Plant processes consist of pretreatment, carousel-type nitrification/ denitrification, clarification and UV disinfection.

In 2006, the plant staff documented increased suspended solids from snail shells in final effluent because a snail population had proliferated in the UV chambers. Plant superintendent David Drobiak asked for assistance from the DEP.

DEP biologist Joe Nestico had suggested that the high snail population appeared to result from an available food source: algae growing in the UV chamber. He suggested regular cleaning of the algal growth on the clarifier launders and weirs and washing of the UV chambers to minimize or eliminate the snails.

The plant’s original design made allowance for algae control, and a conventional brush-type system was installed for the two secondary clarifiers. However, Jewett City found that despite the brush system, algae grew on the scum baffles, weirs, brackets and other areas of the launder walls. This algae sloughed off and settled in the UV chamber.

The sloughed algae appeared to enable the tiny snails to flourish and multiply, while also seeding the UV chamber and promoting colonization of attached algae on the chamber walls.

From brush to spray

The pilot study involved replacing the brush system with a spray system to control the attached algae and reduce the snail infestation. Nestico had been contacted by about an automated cleaning system as a potential solution to the snail and algae problems.

The automated cleaning system is a non-contact cleaning approach using non-potable plant water to form a rotational spray. The waterjet action allows uneven and porous surfaces to be cleaned while minimizing the amount of water used.

The rotational spray acts as a lance, cutting away at the algae growth or debris attached to the tank surfaces. A programmable timer fully automates the process. Nestico wanted to test whether the spray system would control the attached algal growth in the secondary clarifiers and so reduce or prevent UV tube fouling and snail proliferation. The parties also expected the spray to minimize or eliminate regular manual cleaning.

Testing the concept

In fall of 2006, a spare secondary clarifier was retrofitted with the spray system and configured so that only that tank would feed clarified effluent to the UV chamber. Nestico,, Mechanical Solutions (local representative from South Windsor, Conn.), Jewett City WPCF, and several local water pollution control facilities donated staff for the removal of the brush system and installation of the spray system.

During the first year of operation, the plant water system that provides the 30 gpm/65 psi flow to the washer system was found to have a cracked pipe that for some time before and after its discovery limited cleaning effectiveness.

However, even with flow and pressure below specification, the spray system delivered better cleaning, less attached algal fouling in the UV chamber, and an observable reduction in the snail population. In addition, manual cleaning was required only once per quarter instead of twice per month.

During the pilot study, a few mechanical issues developed with the spray system, but corrected them, and the company used what it learned to improve the system design.

Although the washer removed the majority of attached algal growth and regrowth, a residual black growth remained in some cases. This was found to be dead coccoid green algae — it was a cosmetic issue and did not appear to be a food source for the snails.

The parties determined that the best way to address the remaining debris was to add a booster pump to increase the spray system pressure to 120 psi. Testing showed that the booster pump would provide the desired result.

Effective control

Pilot study results to date show that the facility has seen a significant reduction of snails in the UV chamber, and elevated effluent suspended solids from snail shells is no longer a concern. Facility staff members no longer manually clean the clarifier or UV chamber on a regular basis, and the black residual from the dead coccoid green attached algae is significantly reduced.

Although additional time is needed to document the sustainability of the system, it appears to date that the spray cleaning is a viable and effective process to control or eliminate attached algal growth in secondary clarifiers. It also appears to be effective in snail population control in UV disinfection chambers, and in greatly reducing staff maintenance time in facilities with UV disinfection.

About the author

David Drobiak is superintendent of the Jewett City (Conn.) Wastewater Treatment Plant, Joseph Nestico is a senior environmental analyst with the Connecticut Department of Environmental Protection, and Shawna Gill is CEO of Inc., a manufacturer of automated cleaning systems for wastewater treatment plants based in Beaverton, Ore.


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