Current PWPF

Potable Water Production Facility

An aerial view of a current water treatment plant (PWPF).

The City’s current Potable Water Production Facility, as shown above, can treat up to 24 MGD on a peak day. This facility is considered a conventional water treatment facility. In recent years, the City has added a new process to feed Powdered Activated Carbon (PAC) into the raw water to address taste and odor issues. In addition, the City constructed a state-of-the-art chemical feed system in 2015 where alum, lime, sodium hypochlorite, a corrosion inhibiting phosphate, fluoride, and polymers are added to the water during the treatment process.

Water Treatment Process

After the raw lake water is withdrawn from Lake Lanier, it is conveyed to the PWPF via a network of pipes. The raw lake water enters the PWPF through a Parshall flume and the volume of incoming flow is measured as it passes through the flume by a sonic flow meter.

After flow measurement, the raw lake water is rapidly mixed with aluminum sulfate, hydrated lime, and polymer. These chemicals cause the mud and other particles in the lake water to coagulate and form “floc” particles in the flocculation chamber. A non-mechanical, over-and-under baffle system is employed to enhance the flocculation process. After the flocculated water leaves the flocculation chamber, it enters the sedimentation basin. The flow of the water is slowed in the sedimentation basin to allow the newly formed floc particles to settle by gravity to the bottom of the tank. At the effluent end of the sedimentation basin, the clarified water is skimmed from the surface of the basin via saw-tooth weirs. The clarified water is then allowed to flow by gravity through the facility’s state-of-the-art, multimedia filters. These multimedia filters are constructed of anthracite coal, well-graded sand, two grades of garnet, and three grades of gravel. The filter media is supported by a stainless steel, folded plate underdrain

Periodically, each filter must be backwashed to rid the media of the soil and chemical particles that become embedded in the filter during the filtering process. The backwashing process is divided into two separate stages. During the first stage, compressed air is forced up through the media for several minutes in order to scour the media. During the second stage, air scouring is halted and purified water is forced up through the media to wash away the once embedded soil and chemical particles. The wastewater generated by this process, called backwash water, is then pumped to the solids management facility for further treatment before it is released to a creek that eventually flows back into Lake Lanier.

After the water is filtered, it is collected in the Pipe Gallery and flows through the Post-Flash-Mix where fluoride, chlorine, phosphate, and lime is added. After final chemical addition is completed, the water is stored on-site in one of the facility’s clear-wells.