Working Out the Bugs to Maintain a Healthy System

by Dennis Totzke

The workhorse of the industrial wastewater treatment field is aerobic treatment, a biological process that combines the nearly invisible efforts of millions of microorganisms with dissolved oxygen (DO) to purify industrial plant discharges. Of the thousands of industrial biological treatment systems operating in the United States, close to 90% are based on the introduction of air (containing DO) to the liquid waste under certain conditions.

To make these systems operate properly, one must control the amount of air added, the length of time the microorganisms are allowed to mix with the waste and air, and the number and type of microorganisms performing the work. So what are the ideal conditions you should strive for? Here are a few to remember:

TEMPERATURE

Aerobic bugs are like us, they work best over a preferred temperature range. When the environment is too hot or too cold, activity and productivity decrease. For aerobic microorganisms, 50-100 ºF is a common survival range, but 80-90 ºF is preferred for optimal wastewater treatment. Unfortunately, operators can rarely control temperature.

pH

The pH of the wastewater undergoing aeration is rarely constant: it changes as the amount of aeration changes, shifts with weather/atmospheric pressure changes, and adjusts to the biological conversion of organic acids and other substrates to carbon dioxide and more bugs. With that in mind, a range of 6.5 to 8.5 s.u. is survivable, though the preferred pH would be around 7.0 s.u. Operators can adjust the pH of an aeration basin with sodium hydroxide or other chemicals, but at a big jump in operating costs.

BUG POPULATION

The quantity and quality of the aerobic microorganisms in an aerobic treatment system dictate the performance of the system.  Termed “mixed liquor total suspended solids (MLSS),” it is important to match the quantity of bugs to the amount of food. Too many bugs can result in excessive aeration costs and poor effluent quality. Too few bugs can lead to odorous conditions and poor effluent quality. At the same time, the quality or age of the microbial population can affect treatment system performance. It is good practice to routinely observe the aerobic biomass under microscopic conditions to quantify the types of bugs and their relative populations. If you cannot do this onsite, contact ATI for information on offsite testing.

MIXING

It is easy to forget this one. If the bugs are not kept in intimate contact with the food and the DO, system performance will suffer. Proper mixing is necessary to maintain good bug-to-food contact, minimize the settling and/or floating of biomass and food, and prevent significant short-circuiting within the aeration basin. This can be monitored by measuring the DO, temperature, and/or MLSS concentration at various depths and locations throughout the aeration basin. Also, dye studies can be conducted to quantify the average hydraulic retention time, the degree of short-circuiting, and the percentage of dead space in an aeration basin.

DISSOLVED OXYGEN

As rule of thumb, a system should contain two or more mg/l DO in the main part of the aeration basin. This is generally high enough to ensure a positive DO in the corners and lower depths of the aeration basin. The actual DO in the mixed liquor can vary with influent load, water temperature, rate of biodegradation taking place, and the efficiency of your aeration devices.

The optimization of the DO in your system can be used to cut costs, as the electrical energy required to provide it is one of the major operating costs of an aerobic system. DO can be measured frequently with handheld portable DO meters, but be sure to clean and calibrate them frequently. Online DO instrumentation can be linked to the control of the aeration blower VFDs, thus minimizing the amount of aeration HP expended as load and/or performance vary.

NUTRIENTS

Since we are talking about a biological process, it makes sense that the bugs require certain compounds to optimize their growth. The most common of these – nitrogen as ammonia, phosphorus as orthophosphorus, and iron – are generally required in a ratio of 100:5:1:0.5 relative to BOD. Less commonly deficient but of concern for specific industrial wastes, are elements such as cobalt, nickel, copper and selenium. Micronutrient blends are available from chemical suppliers that provide them in small, dissolvable packages. Contact ATI for information on such suppliers.


ATI has been involved in hundreds of aerobic treatment systems, and in our experience, most aerobic systems perform acceptably, but not necessarily optimally. They are robust, adaptable and seemingly well understood, such that they are often taken for granted. With proper testing and tracking of data over extended periods of time, aerobic treatment systems can be optimized to achieve maximum performance at minimum operating costs.

Our experience has afforded us a solid understanding of the complex realities of aerobic treatment. Feel free to contact Dennis Totzke at (262) 784-7690 or This e-mail address is being protected from spambots. You need JavaScript enabled to view it for additional information or to learn more about offsite testing, dye studies, or anything related to aerobic treatment.