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Technology New Insights Into Waste Lagoon Water Mixing An age-old treatment technique gets a technology update. by Jim Dartez M an-made lagoons have been used worldwide for the treatment of domestic and industrial wastewater, it seems, forever. All waste lagoons were first designed and thoughtfully planned considering their locations, soil makeup, and waste reduc- tion processes prior to being implemented. The biological process of organic waste reduction is always a consideration of waste lagoon designs. There are three biological concepts considered for waste lagoons – • Anaerobic lagoons, usually 10 feet deep at a minimum, have no dissolved oxygen in the lagoon water. Acid and methane bacteria work together to convert complex organics eventually to gases. There is very little, if any, mixing in these lagoons, and they are often used as an initial diges- tion process with aerobic lagoons downstream. • Facultative lagoons are more shal- low and were conceived to allow for anaerobic digestion in the bottom sediments and to allow for aerobic Algae blooms become prevalent with noxious filamentous and blue-green clusters in the summer. Even in mechanically aerated lagoons, sludge buildups will appear. In all cases, odors become apparent, at times, and effluent water quality is negatively affected. Sludge in waste lagoons shortens the life of the lagoon and causes odors, extremely high algal blooms, and ... eventually resulting in the degradation of effluent water quality. bacteria to consume the liquid and gaseous intermedi- ate organic products in the water column. Surface water movement supplies the only water movement, and daytime algae are the oxygen source for the aerobic bacteria. • Aerated lagoons are deeper, often up to 20 feet in depth, but mechanical and diffused air systems continu- ously provide dissolved oxygen to aerobic bacteria in the sediments and water column. The aerators are also the source of mixing in the lagoon. Sludge Buildup In Lagoons This discussion will concentrate on facultative and aerobic waste lagoons. Once these types of lagoons reach 5 to 20 years of age, depending on their surface area, owners begin to recognize problems, especially with odors and effluent water quality. In facultative lagoons, operators will begin to notice that sludge is building up around the edges of the lagoon, especially near the influent, and often the effluent. 40 Water Online The Magazine, Wastewater Edition ■ wateronline.com Because we can't actually see inside these lagoons, what we don't realize is that sludge is not only piling up around the influents, effluents, and sides of the lagoons, but long "ripple" type mounds of sludge are building up all over these aged lagoons. The sludge mounds also occur in mechanically aerated lagoons but for a different reason than the rippled, non-aerated lagoons. The cause of these mounds of sludge is poor mixing. In the case of the facultative lagoons, there is never enough flow generated from surface wind action to cause the sludge to level off on the bottom. In fact, the ripples are an expansion of the types of ripples one sees in the surf zone near the shore on a beach. But the mounds of sludge in a lagoon are much larger in size, and made up of very fine silt. Aerators As Mixers In the case of aerated lagoons, depending upon the type of aeration, the sludge mounds form around vortex and vertical splash aerators, in front of aspirating and paddlewheel aera- tors, and along the sides of the hoses in diffused air aeration systems. While all of these aerators provide dissolved oxygen to the water, their water-mixing capabilities are stunted or defeated by the mounds of sludge their high horsepower invokes on the fine silt that lies underwater. There is no question that some of the high horsepower aerator designs move water, thereby somewhat mixing the water; but, when water flow is deflected by mounds of sludge on the bottom, or the bottom itself, the forward momentum of the water flow has a tendency to deflect backwards, thus stunting forward flow and reducing efficient mixing. For diffused air hose and uplift or vertical splash aerators, they merely lift water, thereby dropping the solids particles directly beside the ris- ing bubbles, and these particles become the basis for sludge