Cold water slows biological treatment in two separate ways, and a plant can be affected by either one or both. The first is that the organisms grow more slowly, so the recovery of a damaged population takes longer in the cold than it would in summer. The second is that nitrifying organisms are affected more than others, so a plant that handles organic load reasonably in winter can still miss its ammonia limit. The practical consequence is that the dosing regime has to change in the cold, not simply continue unchanged from the summer.
Product referenced in this article
Wastewater Treatment Bacteria JY-C004 — application notes, specification table and document downloads for this range.
Two different cold effects
The first effect is general. Below about 20 degrees Celsius, the maximum specific growth rate of most wastewater organisms falls, and a population that would double in a matter of hours in summer takes considerably longer. The consequence is a slower return to a healthy state after any disturbance: a plant that recovers from a shock load in three weeks in summer may take six in winter, and the extended period is still producing at reduced capacity throughout.
The second effect is specific to nitrification. Nitrifying organisms grow more slowly than heterotrophs even at optimal temperature, and they tolerate a narrower range of conditions; as a result they are the first to fall behind and the last to return. This produces the familiar pattern of a plant whose organic removal looks acceptable through the winter while its ammonia stays high, and it is why an ammonia exceedance in cold weather should not automatically be read as a dosing failure.
What cold actually does to the process
Two further consequences matter operationally. The rate of the biological reaction drops, so the same amount of load needs a larger volume of active biomass to be treated. And the rate at which the process responds to a change in load drops with it, which is why a plant can be operating acceptably and then breach within a day of a load increase that would have been absorbed in summer.
There is also a physical effect worth separating from the biological one. In cold water the gas transfer rate is lower, so aeration is less efficient at the same air flow. A plant can therefore have less oxygen available to the biology than it appears to, and the dissolved oxygen reading at the basin tells you about the mixed liquor rather than about the transfer capacity. Where the aeration system is marginal in summer, it becomes the limiting factor in winter before the biology does.
What helps and what does not
What helps is straightforward. Increase the active biomass ahead of the cold period rather than during it, because a population takes weeks to build and a cold period is not the time to start. Reduce the load where there is operational scope, since the same treatment capacity handles less in the cold. Keep the aeration system clean and check the transfer rate before the season rather than during a exceedance. And lengthen the retention time if the process configuration allows it, which is the cheapest lever available when a limit is being missed.
What does not help is dosing more of the same product on the assumption that more is always better. The population that is slow to establish is slow because of temperature, and adding a large single dose into cold water does not accelerate the growth rate. A culture added ahead of the season, in conditions that allow it to establish, is worth considerably more than the same total added during the first cold week.
Planning for the season
The useful work is done in the two months before the temperature drops. Confirm that the biomass is healthy while conditions still allow fast growth, and increase the solids concentration then rather than in January. Review the aeration capacity against the expected summer-to-winter oxygen demand, because air capacity is fixed and demand rises when biological rates fall. Establish what your ammonia limit becomes at the reduced rate, so the operating target is realistic rather than aspirational. And decide in advance what the trigger will be for a culture dose, so the decision is made on a measurement rather than on a date.
One honest warning. Any figure quoted for cold operation is specific to a temperature range, and a supplier quoting a rate without asking what your winter temperature actually is is quoting a number from a bench study. Send us the monthly temperature range, the current solids concentration, the retention time and the ammonia figures you are seeing, and we will tell you what we would expect and what we would want to measure.
Frequently asked questions
- Why does ammonia rise in winter when organic removal looks fine?
- Because nitrifying organisms are affected earlier and recover later than the organisms responsible for organic removal. Nitrifiers grow more slowly even at optimal temperature and tolerate a narrower range of conditions, so in cold water they fall behind first. The plant therefore keeps removing organic load reasonably while its ammonia performance declines, and the two do not move together.
- Does dosing more bacteria solve cold-weather performance problems?
- Not by itself. The population is slow to establish because of temperature, and a large single dose into cold water does not raise the growth rate. A culture added before the cold period, in conditions that allow it to establish, is worth considerably more than the same total added during the first cold week.
- Which operational changes help most in cold weather?
- Increase active biomass ahead of the season rather than during it, reduce load where there is scope, clean and check the aeration system before the season, and lengthen retention time if the configuration allows. Retention time is usually the cheapest available lever when an ammonia limit is being missed.
- What temperature data do you need to advise us properly?
- The monthly temperature range at the site, the current mixed liquor solids concentration, the hydraulic retention time, and the ammonia figures you are currently seeing. A supplier quoting a rate without asking what your winter temperature actually is is quoting a number from a bench study rather than from your plant.