Fixed Film Treatment Systems

MIL-HDBK-1138

the system in the air flow; conversely, mechanical mixing systems

will have significant temperature decreases across the aeration

basin as cold air is mixed into the activated sludge. Take these

temperature changes into consideration when selecting or changing

aeration methods in a cold climate. Avoid ice buildup across the

surface of the basins. This buildup will limit the oxygen

transfer and may interfere with mixing equipment. Removing

floating scum before winter will help prevent this extra material

from freezing in the aeration basin.

In cases where the temperature is affecting the

process, changes to the process control are required. Process

adjustments would include increases to the mean cell residence

time (MCRT) or a decrease in the food-to-microorganism ratio

(F/M) to maintain the same process performance and loading rates.

Ice buildup around mechanical aeration equipment will

also affect safety and may overload equipment if it attaches to

equipment surfaces. Continuous operation of mechanical mixers at

low speed or intermittent operation will reduce the oxygen

transfer and reduce ice buildup. Monitor oxygen concentration

closely if this method is attempted. Heat tracing the components

may also be helpful.

7.4.2

Fixed Film Treatment Systems. In trickling filter

plants, the potential for icing is very high. In winter, turn

off the draft in forced-draft systems unless a source of warm air

is available.

Note: The forced draft systems are used primarily when

the outside air is less than 5F (3C) above or below the process

liquid temperature. If ice forms within the unit, flood it to

melt the ice. Take care when flooding the media, and be cautious

of structural loading as well as leaking and overloaded pumping

systems, gates, and valves. Ice can form on the top of the

media, which may affect the distributor arm. Covers are advised

in extremely cold areas to prevent this situation.

Rotating biological contactors (RBCs) are usually

covered to control odors, but keep the covers in good condition

to keep the heat within the wastewater. If the RBCs do not

already have covers, install them to keep the media from freezing

and affecting the shaft with additional loading and potential

failure.

7.4.3

Lagoon Systems. Aerated lagoons will typically

freeze during the winter. Remove mechanical mixers to prevent

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