Just water down the drain?

A case study of an Ann Arbor plant.

There are several ways to treat municipal wastewater, but they all follow the same general principles. At the Ann Arbor wastewater treatment plant, several remote lift stations throughout the city help keep the wastewater flowing. Ultrasonic level detection controls the pumps, and remote monitoring ensures proper operation. The plant's design allows wastewater to flow through the influent, displacing and thus discharging the effluent from each tank. Archimedes screw pumps lift the high volume of water-providing hydraulic head through the process.

Wastewater coming in should be gray, have a milder-than-expected smell, and look like dirty bath water. We screen the wastewater to remove large items, which could foul up the process.Then we remove the grit. Grit would not only add inert solid bulk, but also would scour and wear away any other equipment it runs through.

We want to maintain a relatively constant flow to avoid upsetting the process, but the flows coming into the plant vary greatly throughout the day. As the incoming flow increases above the daily average flow, we direct the excess to the equalization and retention tanks.Then as the flow drops off, we send the retained flow back to the lift screws and into the plant. The flows are controlled automatically; the operators just set the desired plant flows to keep everything in balance, so we neither run out before flows pick up, nor have too much in reserve from day to day.

In Ann Arbor we have six different passes, two in the older west plant and four in the newer, larger-capacity east plant. Having six passes allows us to take a pass down for maintenance-a primary treatment tank, an aeration basin, and a secondary treatment tankwithout having to shut down the plant.

During primary treatment, the larger, heavier solids settle out. Then we remove them from the bottom of the tank. If we do not remove enough solids, the tank could go septic, creating smells and providing too much biological oxygen demand (BOD), or food for the bacteria, perhaps overloading the process. Removing too many solids could result in the removal of too much BOD for the bacteria in the aeration tanks.

The wastewater then flows into the aeration tank, mixing with activated sludge from the secondary treatment tanks. Blowers provide air for mixing and oxygen. As the bacteria eat up the BOD in the aeration tanks, the material mixes around or flocculates.These floes will settle in the secondary treatment tanks. A properly mixed, healthy aeration tank should be light brown with an earthy smell-not a foul smell at all from this point downstream.

The water flows into the secondary treatment tanks. The solids sink-pumping from the bottom (most as return activated sludge) to the head of the aeration tanks to mix with the primary effluent. We remove wasteactivated sludge to maintain proper sludge inventories and to maintain a proper sludge age. We must have enough bacteria to metabolize the BOD but must waste enough to keep the sludge from getting old enough to break down and decay. We maintain a sludge age of about ten to twelve days. We change it according to plant parameters, time of the year, and plant experience.

The clear water flows over the top of the weirs. In our Ann Arbor plant the water then flows through sand filters. It is like a standard sand pool filter, except the filters consist of 12 pairs of cells, each about the size of a swimming pool.

The water then goes through the ultraviolet (UV) system-banks of specially designed UV bulbs submerged in the water. The ultraviolet light is intense enough to kill any pathogens that happen to make it through the plant, eliminating dangerous chemicals. Also UV treatment has no residual chemicals added to the water. Under normal operations we discharge it to the river knowing it is cleaner than the river itself.

See related story on page 48.

Behind the byline

Brian A. Miller is an instrument technician for the city of Ann Arbor WWTP,a 29.5-million-gallons-per-day wastewater treatment plant.

Copyright Instrument Society of America Feb 2004
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