BULK VOLUME FERMENTER DESIGN/OPERATION TIPS
PRETREATMENT: Sample recommended units: ¼” coarse screen and DAF (dissolved air flotation cell)
TEMPERATURE: process temperature is key. One will want to avoid seeing an anaerobic treatment plant for wastewaters that contain grease, such as meat processing, milk, cheese, etc., operate at less than 32C. If client or engineers insist, they have to deal with the consequences of designing and operating at lower temperatures. First , a larger reactors. Second, most of the grease will float to the top and form a scum layer that in some cases with meat slaughtering operations has approached six ft (2 m) in depth. This scum layer is very difficult to breakup especially if the reactor is covered with a membrane type material. These problems may occur at 32C, but to a much lesser extent. Therefore stick to and maintain the higher temperature range. Below 20°C removals are simply settling, hardly any anaerobic biological process contribution.
MIXING: make sure you will be able to explain how you will keep and maintain content well mixed in the reaction zone.
NO SOLIDS CHEAT and NO DUMPING WAS INTO FERMENTERS
One thing we recommend strongly is that you should NOT put waste activated sludge back anaerobic digesters unless it has been specifically designed for solids such as classical sludge digesters. This tends to fill the digester with inert solids and over time will fill the lagoon/basin or tank with inert solids or dilute the activity of the anaerobic biomass.
MAKE SURE EVERYTHING'S OK INLET DISTRIBUTION AND RANS LATERALS
The inlet and RANS laterals should be placed on blocks -- concrete should be OK -- to which the laterals should be fastened. They should be heavy enough to avoid flotation of the pipe. Vertical concrete walls around the top of the basin are used to attach the cover.
TYPE OF COVER
That's your decision. There are several options for lagoon covers. Some are double layer; others are single. We personally prefer the type of cover that floats on the surface with gas collection at the sides and ends. This type of cover avoids wind damage. We have actual plant records showings some problems with covers that allow gas collection under the cover.
The gas is channeled to the perimeter via lateral floats. The floats create a high point and the rain trench creates a low point. A micro gradient is formed between the two structures. Rainwater flows downward into the trench and gas bubbles formed under the cover travel along the gradient to the high point where they are pulled to perimeter piping via negative pressure. The perimeter piping terminates at a draw-off pipe (preferably through the slope and not the cover) to a flare or genset.
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