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During the winter months, aerated digestor capacity is sometimes maxed out due to cold weather limitations, decreased activity, and land application limitations. Many plants often forget that the digestor has the same "Critical 5" components as the aerated or secondary portion of the wastewater system.

The only difference is that you are starving the bacteria in the digestor to decrease solids. The bacteria in the digestor still need pH control especially since CO2 and respiration continue. N and P are usually not a concern, due to the re-release of nutrients from the dying bacteria. Oxygen is critical and many plants do not make sure that mixing and D.O. levels are maintained and digestors often turn septic and create many of the filament problems found in a plant due to the fact that the supernatant is returned to the front of the system. This winter, when biological activity starts to decrease as temperatures fall, make sure that all 5 of the critical parameters are met in your aerobic digestor.

Fall rains, especially in the south during Hurricane season can cause hydraulic washout in a system if excessive. Make sure that you check the beds in your clarifier carefully. Too high a bed or too old sludge will increase the chances for solids washout with high flows.

Spring or fall turnover also may contribute to temporary odor control issues in wastewater lagoons. Turnover occurs as the lagoon is heating up in the spring or cooling down in the fall. During turnover, water from the bottom (which is high in odor) comes to the top and water from the top moves to the bottom. Increasing the air or D.O. and mixing in the lagoon can help shorten the turnover period.

Why does Spring and Fall turnover happen? The density of water changes as temperature changes. Water is the most dense at 39°F. Both above and below that temperature, water expands and becomes less dense. This means that in the spring, just before the ice melts, the water near the bottom will be at 39°F. Water above that will be cooler, approaching 32°F just under the ice. Warm water is lighter than cool water, therefore cooler water sinks. This causes a complete turnover in the system as the weather changes. Wind action and increasing density cause this surface water to sink and mix with the deeper water. Normally clear water may become murky without rain causing it as solids that were mixed in with the bottom layers of water rise to the surface. You may notice a stale, rotten odor as the unoxygenated or spetic deeper water comes to the top also.

The sequence of events leading to Fall overturn include: (1) cooling of air temperature and stronger winds (2) the cooling of surface waters; (3) density changes in surface waters producing convection currents from top to bottom; (4) circulation of the total water volume by wind action; and (5) vertical temperature equality. The overturn results in a uniformity of the physical and chemical properties of the entire water mass.

Another thing to consider with the coming cooler months- With the decrease in temperature, light and residual N and P, algae growth will decrease, but if there are high levels in your clarifier, they will slough off as they die and can clog pumps, contribute to high TSS in the effluent and generate false BOD. Algae control on the weirs of the clarifiers or other parts in the system is critical. Algae can lead to a false BOD reading. Each milligram per liter of algae in the effluent exerts approximately 0.5 mg/L of BOD5 as a rule-of-thumb. Algae can also impact D.O. levels in receiving streams or in the system. Lower levels of dissolved oxygen as a result of algal blooms causes fish kills in the receiving final body of water. Weir maintenance is critical in spring and summer months, as well as in the fall and winter.

Biological activity increases and decreases with temperature changes. For every 10° F- change, the activity of the bacteria changes one logs growth. Make sure that you are prepared for the decrease in activity as the temperature cools down. D.O. requirements will decrease as activity goes up, so you may be able to save some electricity and run your aerators at a lower speed. Nutrient addition is applicable at your plant will decrease. MLSS values will need to be adjusted and possibly raised in your system. Clarifiers beds need to be monitored and adjusted and they might need to be higher, since you will need to carry a higher inventory if your plant gets serious impact from cold weather. pH will need to be monitored as activity decreases. Nitrification may slow down. If this is critical at your plant, you need to be very aware of this.

Some plants, especially industrial facilities that are out in the middle of nowhere and get extremely cold temperatures, often use steam injection directly into the basins in order to keep the biological activity up. The steam will kill the bacteria that come in close contact with the direct injection, but the overall increase in temperature across the system significantly increases biological activity.

Many plants can also experience an upset condition with dispersed growth and filamentous bulking every winter when colder temperatures promote the growth of some filaments such a Microthrix parvicella.

Some plants can experience a loss of chlorine disinfection during nitrification onset, due to a build-up of nitrite. Nitrite has a significant

chlorine demand (one part nitrite consumes one part chlorine) while ammonia and nitrate do not.

Many plants with lagoons system that do not have the ability to increase their RAS and decrease their WAS in order to build up a MLSS in the system turn to bioaugmentation during the winter months. This is a very cheap, efficient way to increase the amount of bacteria in the system and increase the degradation capabilities of the system.

The bacteria are fed based upon flow, temperature and BOD loading and are only used to take up the slack that the existing bacteria in the system cannot handle. They are then discontinued during the warmer months.

Bioaugmentation is also used to supplement tank remediation, industrial facilities with activated sludge, but with extreme changes in flow and BOD loading that are so significant that the RAS cannot be changed quickly enough to help with the additional loading.

Some Plants have sludge drying beds. Winter is very hard on these systems. Cold weather can freeze the solids and make dewatering harder. Also many plants that land apply their biosolids end up with huge inventories during the winter months, since the biosolids cannot be land applied.

Optimization of your polymer program is necessary to make sure that you do not have wet cake solids. Check the speed of the belts, the rinse jets, the polymer program as well as the application points. There are many ways to optimize your solids dewatering programs.

We made recommendations to one customer who beat on his polymer rep to do some more jar testing. Your polymer program in the winter might not look anything like your polymer program in the summer months. We had them test a dual program since they had an anaerobic digestor and needed a bit of coagulant prior to the flocculant. They changed their program and cut their costs in half! Make your sales rep work. That is part of his job.