Lagoons

• Lagoon Aeration
• BOD Reduction
• Mixing & Equalization
• Energy Savings
• Sludge Reduction
• Ammonia
• Phosphorus

Enhanced Sludge Reduction in Lagoons

A grease rendering company operates a total treatment plant that uses both physical and chemical system components to recover FOG products from food processing wastewater that is collected and brought to their processing facility. The company converts these FOG materials into saleable by-products that are added to various animals feed products for their protein value. As the volume of received materials increased the equipment and process stayed the same. This meant that as aging equipment required more maintenance more solids have been diverted to lagoon. Over the years of increased received materials the solids in the lagoon have increased to a three feet depth in a six-foot deep five-acre (5) lagoon adversely impacting plant operations with the reduction in hydraulic retention time in the lagoon.

This lagoon had been operating with four (4) 30 hp AireO2 surface aerators that were not providing sufficient mixing and aeration to impact any sludge reduction, in fact, they contributed to sludge accumulation by pushing the sludge toward the periphery of the lagoon along the banks. Over time as the AireO2 surface aerators failed the operator decided to replace them with Ultra V venturi aerator systems. Each system consists of a four-inch 15 hp Gorman Rupp self-priming pump [Model 84A52B] with a four-inch (4”) venturi aerator and a NEMA-4 Simplex control panel.

The venturi aerator systems were installed with an innovative suction manifold that would influence a wide area. The manifold was designed so that the height of the manifold could be placed in the sludge layer and would bring the sludge into the pump and venturi aerator where it would be processed and aerated for digestion. Additionally, using this innovative suction manifold ensured that the sludge would be continually re suspended in the aerobic zone.

The following photo shows:

1. The Suction Piping Manifold,
2. The Risers that can be constructed at required elevations for proper operation of the lagoon
3. The screens (in red) 

A principal benefit of using this suction manifold is that a lagoon can be operated either in an aerobic or facultative mode.  In the facultative mode aerobic and anoxic zones are established with the height of the risers on the suction manifold intakes and the elevation of the discharge piping.

In the first year of operation, the operator measured a full one-foot reduction in sludge volume down to the two-foot level. In the second year there was another one-foot reduction in sludge volume to the one-foot level. In the third year of operation the bottom of the lagoon consisted only of light fluffy solids that resembled flock, and even today five years later the only light fluffy flock-type materials remain on the bottom.

The pump provides all the kinetic energy for the venturi unit and also provides the energy for mixing and equalization in the lagoon.  By offsetting the suction and discharge piping for each system, only anoxic water and solids are pumped into the venturi unit thereby increasing the oxygen transfer, as it requires less energy to add dissolved oxygen into anoxic wastewater than wastewater that is already partially oxygenated.

An added advantage of using the venturi system is increase safely.  When servicing the AireO2 units the operators needed with a boat and a crane to lift them out of the lagoon, while servicing the venturi is easy as the entire system is readily accessible on the shore of the lagoon eliminating the need for cranes and boats.  This feature alone is a principal reason operators like the venturi system. Depending on the pump selected lubricating and inspecting the pump can be accomplished in minutes.

Different operators will configure the equipment as necessary for the flow. Some operators have set units across from each other to operate as a “plug flow” aerobic lagoon.  This type of plug flow layout maximizes hydraulic retention time and lower BODs are achieved on the effluent side usually using a baffle curtain for final stage quiescence to act as a clarifier.

Over the last six years of operation a total of six (6) venturi aerator systems have been added to replace the original four AireO2 units. These additional units have been added as the plants daily volumes have continued to increase along with higher levels of BOD and FOG loadings. Because each system is a stand alone with its own suction manifold and discharge piping the operators have found that adding a new venturi aerator system can easily address the increase in oxygen demand from the higher organic demand levels and they can be turned on or off as required so that not all units run all the time.

The original four AireO2 units had 30 hp motors for a total of 120 connected hp operating at 120. The six four-inch Ultra V venturi aerator units have only 90 connected hp operating at 78 hp. There is a reduction of 42 hp and the accompanying energy savings, but more importantly is the reduction in sludge, which increases lagoon volumes for longer HRT that ensures lower BODs and better effluent clarity (TSS).