There are 5 critical measurements that should be monitored and controlled to effectively run a biological treatment plant efficiently; Temperature, Dissolved Oxygen, Ammonia, Ortho-phosphate and pH.

PARAMETER                                 ACCEPTABLE                        OPTIMUM
Dissolved Oxygen                           >0.5 mg/l                          1.0 - 2.0 mg/l
Temperature                                  40 - 95◦  F                        77 - 95 ◦ F
pH                                                7.0 - 9.5                            7.5 - 8.5
Ammonia Residual                          1.0 - 3.0 mg/l                    2.0 - 3.0 mg/l
Ortho-phosphate Residual                0.5 - 2.0 mg/l                   1.0 - 2.0 mg/l
Residual should be measured in the final effluent, unless limited by permit restrictions.

As silly as these may seem or appear to be just general information, they are extremely critical. 90% of all audits we conduct have at least one or more of these parameters in the wrong range. These are the easiest parameters to control at your wastewater treatment plant and some of the most critical ones. The bacteria do not care about any tests you perform, (i.e. sludge judge, MLSS, F/M etc.) parameters you measure or implement as long as these Critical 5 are in the correct range. Biomass quality and final effluent results will be impacted.

Cold weather can sometimes be a problem for many plants due to permit restrictions and decreased biological activity. Many plants experience a significant drop in biological activity due to the temperature levels decreasing. Biological activity drops one log level for each 10-degree drop in temperature. This can significantly impact the amount of BOD loading that the biomass can handle effectively.
What is the "plus one"?

Alkalinity is sometimes considered the plus one but only for plants that require the additional step on Nitrification.

Nitrification

Nitrification is a sensitive process that must coexist with the carbonaceous BOD removal process. There are many stresses that can adversely impact nitrification before the BOD or TSS removal efficiencies are affected. Biocides that are used in the cooling towers can contain chemicals that are toxic to nitrifiers. Gluteraldehyde is very toxic to nitrifiers and it tends to stay in the activated sludge. It is absorbed by the biomass. Zinc can also be toxic to nitrifiers. Alkalinity and pH were low at this time. Alkalinity is critical to Nitrification. Nitrifying bacteria

As ammonia is removed it is transformed-For each 1 gram of NH3-N oxidized to NO3, 0.15 grams of new bacteria cells are formed. Most of the NH3-N is used as an energy source. It is used in a non-assimilative way so only a small amount of biomass (sludge) is produced. Nitrification occurs 3-4 times slower than carbonaceous oxidation. Carbon dioxide (CO2) or carbonate is used as the carbon source in nitrification. 4.5 parts of O2 is needed for every part of NH3 to be degraded.

First Conversion (Ammonium to Nitrite)
Nitrosomonas bacteria oxidize ammonium to nitrite via hydroxylamine.
2NH4+ + O2 --> 2NH2OH + 2H+ NH4+ + 1.5 O2 --> NO2- + 2H+ + H2O

Second Conversion (Nitrite to Nitrate)
Nitrobacter bacteria convert nitrite to nitrate.
NO2- + 0.5 O2 --> NO3-