Operational issues at your wastewater treatment plant

posted by: Karen Bonvillain | 5th February 2015
Operational issues at your wastewater treatment plant

By Sam Gates, Environmental Scientist

This article was originally published in City & Town, December 2014 pages 24-25. Arkansas Municipal League publication.

Operational issues at your wastewater treatment plant can be divided in to four problem areas that can affect the wastewater NPDES (National Pollutant Discharge Elimination System) Permits. They are: 1. mechanical/electrical, 2. chemical, 3. biological, and 4. operational.

The NPDES permits are renewed every five years and are regulated in Arkansas by the Arkansas Department of Environmental Quality (ADEQ), which is governed by the Environmental Protection Agency (EPA) Region 6. The EPA and ADEQ get their regulation authority from The Clean Water Act.

Mechanical issues

Mechanical issues refer to when an important piece of equipment is off line due to mechanical or electrical problems. Mechanical can include pumps, screening equipment, blowers, clarifier, and sludge handling equipment, or some other piece of instrumentation for process control or measuring the flow. These also may include electrical problems such as lack of power, loss of phase, power bumps, and lightning strikes, which can cause equipment to be off line due to blown fuses, motors, wiring, and tripped breakers, or damage to the electrical switchgear (breakers, starter contacts, starter coils etc.) motors or wiring.

Chemical issues

Wastewater treatment, whether its in a large facility like Fayetteville’s, seen here from the air, or a small town’s lagoon system, is a complicated and important process.

Wastewater treatment, whether its in a large facility like Fayetteville’s, seen here from the air, or a small town’s lagoon system, is a complicated and important process.

Chemical issues refer to when the pH, ammonia loading, or dissolved oxygen has changed or some unknown chemicals are coming through the plant affecting the final effluent and perhaps the required permit limits. Chemical issues can also include a lack of chemicals that may be required, such as chlorine for disinfection or polymers used for settling and solids coagulation. Also there are chemicals to sequester copper and phosphorus from the effluent.

Chemicals in the influent can also affect areas like biological and operational and are sometimes responsible for killing or shocking the treatment process. These chemicals can be numerous and varied, from detergents, cleansers, disinfectants, herbicides, pesticides, solvents (paint thinners and strippers), oils and greases, paints, and metals. Many new or renewed NPDES permit requirements include monitoring or placing limits on dissolved copper and phosphorus on the wastewater effluent discharged from the treatment plants. These new copper limits come from the results of research showing that even small amounts in parts per million (ppm) can stunt the wing development and growth of the stone fly, a major food source for the small mouth bass, trout, and other fish in Arkansas. Not affecting the NPDES permit limits but of concern on copper limits, Japan has performed research that indicates even very small amounts of copper in drinking water can cause gastronomical distress in babies when the water is used in formulas.

The phosphorus limits come from the nutrient discharge that encourages algae and bacterial growth in the receiving streams affecting water for sometimes miles downstream in the receiving stream. Copper sulfate has been the old standby for many years in sewage lagoon treatment and fresh water supplies (reservoirs, lakes, and plant filters) to control the summer algae blooms that cause Total Suspended Solids (TSS) problems in the effluent. Algae can also cause blinding of the treatment plant sand filters. New higher priced algaecides that contain chelated copper or Sodium Percarbonate (hydrogen peroxide and soda ash) and increase alum or polymers are required, increasing treatment plant costs. Copper can also come from water distribution pipes in a typical household. For many years, it was common practice to ground telephone and electrical wiring to the water piping. This caused very minor leaching of the copper from the household supply piping. Water plants typically are adding Calgon or zinc orthophosphate or some other orthophosphate product to the water to sequester the copper that coats the inside of the pipe. This coating prevents leaching but adds phosphate to the water stream, and therefore, to the wastewater treatment. All of this is a delicate balancing act for the operators at the water and wastewater treatment plants.

Biological issues

Biological issues are where the biomass has been affected and the bacteria are shocked, stunted, or killed back and then begin starting over to rebuild sufficient bacterial numbers to break down and clean the water. The biological treatment biomass consists of a mix of different bacteria, algae, plant material, and other organisms that function together to remove the nutrients and break down the proteins, amino acids, and other waste products in the sewage. This process is described as Nitrification and Denitrification in the treatment process.

Nitrification is the biological removal by oxidation of ammonia (NH4+) by nitrifying bacteria. These nitrification bacteria are called autotrophs or autotrophic bacteria and are primarily two main species—Nitrosomonas sp. and Nitrobacteria sp. They use inorganic carbons such as Co2, carbonates, and bicarbonates. These are very slow growing bacteria, only doubling every eight hours for Nitrosomonas and every 12-24 hours for the Nitrobacter. Heterotrophs or heterotrophic bacteria that use organic carbon compounds to grow and gain energy are the predominant species found in wastewater plants and can double every 20-30 minutes under ideal conditions.

Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In both systems, including the wastewater treatment process, denitrification occurs when oxygen is depleted and the heterotrophic bacteria turn to nitrate in order to use organic matter. Because oxygen is abundant in our atmosphere, denitrification only takes place in this low dissolved oxygen condition. It should be noted that nitrifying bacteria are very sensitive and the following can interfere with their growth and ability to breakdown ammonia products:

  • The ideal pH is 7.6-8.2 but is not required.
  • Nitrification stops with a low pH or alkalinity <5.0.
  • Low dissolved oxygen 0.5 mg/l and nitrification will stop with 2-3 mg/l being ideal.
  • Low temperatures—Nitrification will cease at 41ºF with optimum condition ranges of 82-90 ºF. Nitrification proceeds at about 50% at 61 ºF and 20% at 50ºF.
  • Septicity, related sulfides and other organic acids can interfere with Nitrification.

Operational issues

Operational issues can also be classified as lack of trained personnel, inexperience in the operations, and changes that need to be made in operations during high rain events or seasonal changes due to temperature and loadings. These also include what to do when something unexpected comes through the plant that affects the treatment permit limits. NPDES permits typically have different effluent requirements based on the winter and summer months that discharge to the receiving stream.

Typically, biomass concentration adjustment may be required unless the treatment plant is just an aerated lagoon system. But even with lagoons treatment, ammonia nitrogen, TSS CBOD, chlorine disinfectant, dissolved oxygen, and flow measurements must be monitored and reported. If lagoons are not meeting limits, it must be determined why the lagoons are not performing. Lack of air or lack of treatment capacity could be the answer.

As any manager or operator can tell you, there are many operational issues that need attention every day at the wastewater treatment plant. All of the issues need to be balanced with manpower and budget considerations in order to keep the plant within permit limits. As may be inferred from the above passages, wastewater treatment is very complicated. Our operators are often taken for granted, but they deserve a level of respect that many do not receive.

 


Sam Gates is an Environmental Scientist at McClelland Consulting Engineers, Inc., in Little Rock. Contact Sam at 501-371-0272 or sgates@mcclelland-engrs.com.

Sam Gates is an Environmental Scientist at McClelland Consulting Engineers, Inc., in Little Rock. Contact Sam at 501-371-0272 or sgates@mcclelland-engrs.com.

 

 

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