Visit Lagoonsonline.com and specifically view the following links for cold weather nitrification:
http://www.lagoonsonline.com/richard.htm
http://www.lagoonsonline.com/ripple3.htm
The use of a substrate on which the nitrifiers can attach themselves and increase their number such as the ringlace fixed film system is something to consider.
Depending on sunlight availability, you may cogitate on the use of a solar powered circulation system.E. coli happens!
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