The City of Evansville serves over 600 residents with drinking water from two groundwater wells. To meet EPA drinking water standards, the city’s groundwater was treated at the water treatment facility (WTF) using aeration, gravity filtration and chemical feed for iron, manganese and arsenic removal from their raw water.

Ammonia, present in the groundwater and not removed during the treatment process, caused taste and odor issues as well as increased chlorine consumption in Evansville. These deficiencies in the treatment process were cause for concern amongst City Staff. In addition, safety and health risks associated with an antiquated chlorine gas disinfection system was a concern.  Based on these treatment plant deficiencies, the city decided it was time to correct the problems and thus Apex Engineering Group was retained to assist with improvements to the WTF.

Apex completed a facility plan to evaluate the current WTF where most of the equipment, controls and ancillary items were reaching the end of their useful life. Following the study, Apex decided that a remodel of the existing plant was the most cost-effective plan and the city would reuse the existing WTF building. In order to meet the confines of a limited footprint, it was decided to remove the existing interior floor, leaving only the building’s shell so the newly selected process equipment could be properly configured. The building height was raised to a ceiling height of 19 feet and a new mezzanine level was added to accommodate the required mechanical equipment located above the expanded chemical room. It was paramount during the process design for the newly remodeled facility that the city’s three main treatment concerns be addressed — removal of as below MCL of the existing gas chlorine system and removal of ammonia present in the groundwater supply.

Addressing Gas Chlorine  The city chose an on-site hypochlorite generation system using water, salt and electricity to produce a liquid containing 0.7% chlorine compared to the 100% chlorine gas used previously. This resulted in a significantly more stable and safe disinfectant for the city’s operating staff.

Resolving Ammonia  To resolve the ammonia issue, a packaged gravity filtration system was selected. The filtration system uses a combination of aeration, chemical oxidation (permanganate), polymer, and a combination sand/anthracite filter media to remove arsenic, iron and manganese. Since no disinfectants are dosed prior to filtration, the media can grow nitrifying bacteria, which are key for ammonia removal.

Temporary Groundwater Treatment  A key component of any new treatment facility project is maintaining treatment during construction. Apex worked with the Minnesota Department of Health (MDH) to ensure temporary treatment requirements were met. One of the requirements included a public meeting and extensive pro-active public education campaign which provided ongoing WTF project updates, guidance, health information and test results during and after construction. In addition to the public education, MDH required a flushing protocol of the distribution system during construction and once the new WTF was operational.