Nitrification in waste water treatment is of concern to many facilities. Problems vary from temperature, shock loading, overloaded systems, washout, and other conditions that make permit level nitrification difficult or impossible to meet. A given treatment plant may have occasional problems (related to specific conditions that happen from time to time) or it may have chronic problems. ESI has potential solutions for either case, depending of course on the specifics.
General Information on Nitrification
Nitrification is the biological oxidation of ammonia to nitrate. The nitrification process consists of two steps:
STEP #1 Conversion of ammonia (NH3
) to nitrite (NO2
) STEP #2 Conversion of nitrite (NO2) to nitrate (NO3) Step #1 is performed by the bacterium, Nitrosomonas.
Step #2 is performed by the bacterium Nitrobacter. Various parameters play a key role in successful nitrification. Among these are the following:
Presence of Required Bacteria:
An obvious requirement is the presence of Nitrosomonas and Nitrobacter bacteria. However, their presence or absence may be a function of the other parameters listed below.
Nitrifying bacteria are aerobic bacteria, which means they require dissolved oxygen in order to metabolize, grow and reproduce.
Alkalinity of water is a measure of its capacity to neutralize acids. A variety of compounds, including bicarbonates, salts of weak acids, and hydroxides contribute to alkalinity. When ammonia is oxidized during nitrification, hydrogen ions (H+) are liberated. Alkalinity is needed to neutralize these hydrogen ions. In fact, 8.64 mg/l of alkalinity are consumed for each mg/l of ammonia that is oxidized. Without sufficient alkalinity, the pH of the system will drop, and nitrification will slow down or even stop.
Nitrification works best when the pH is between 6.5 and 8.5. The process slows considerable at pH values outside this range.
Assuming that dissolved oxygen (D.O.) remains constant, nitrification efficiency decreases as the temperature falls. Many plants have severe nitrification problems during the cold winter period. A related problem may occur during hot summer weather. Since DO is less soluble in hot water than cold water, it is sometimes difficult to maintain necessary DO levels in the summer. When this occurs, nitrification may become inadequate during hot periods. Extreme weather (hot and cold) can impair nitrification efficiency.
Simultaneous Removal of BOD:
From a practical point of view, nitrification proceeds better when the concentration of soluble BOD is low (less than 20 mg/l). Therefore, many treatment plants are designed with two stages. The first stage is for BOD removal. The second stage is for nitrification. In some plants, BOD removal and nitrification must occur in the same aeration tank, trickling filter, or RBC. The BOD removal efficiency of a plant is always an important factor in nitrification efficiency.
Growth and Reproduction Rate of Nitrifying Bacteria:
Compared to heterotrophic bacteria, which consume organic compounds, the growth and reproduction rates of nitrifying bacteria are quite low. Heterotrophic bacteria take as little as 30 minutes to reproduce. Nitrifying bacteria take several hours to reproduce. For every gram of organic substrate consumed, roughly 0.4 grams of new heterotrophic bacteria are produced. However, for every gram of ammonia converted to nitrate, less than 0.2 grams of nitrifying bacteria are produced.
Chemical Inhibition of Nitrifying Bacteria:
There are only two main species of nitrifying bacteria, Nitrosomonas and Nitrobacter. There are varieties of chemicals that are known to inhibit these bacteria, such as sulfides and cyanides (among many others). Since there are only two main species involved, nitrifying bacteria are very susceptible to chemically based toxicity, shock or upset.
Many waste water treatment plants (WWTP’s) are subject to discharge permits, which limit the amount of ammonia that can be lawfully discharged. Government permits and regulations influence levels of ammonia and nitrite that may be discharged into streams. The discharge of ammonia or nitrite into a stream, lake or river may cause oxygen to be consumed, thereby lowering the dissolved oxygen concentration and endangering the aquatic ecosystem.
Maintaining nitrification would be relatively simple for wastewater treatment plants if they operated in a “steady state”. However, the desired “steady state” conditions do not exist in the WWTP environment. For example, all treatment plants experience significant changes in daily flow, organic loading, nitrogen loading, temperature, and concentrations of toxic chemicals. The nitrification process can be easily upset and lose efficiency with the numerous changes that occur on a daily basis. Of course, the most severe cases occur when a facility violates its ammonia discharge limit.
As previously discussed, a plant may nitrify only some of the time, incompletely, or not at all. Many factors may be involved when nitrification is inadequate.
Among the most important are:
- Inadequate physical plant design for effective nitrification.
- Inappropriate biological conditions.
- Occasional process upset due to toxic chemical shock.
- Occasional process upset due to hydraulic shock
- Difficulty in establishing nitrification when variations in seasonal permits exist.
- Loss of nitrification during extreme temperature seasons.
The degree of improvement from use of The ESI Nitrification System
varies depending on which specific factors are causing the problem. The primary benefit of the ESI Nitrification System is meeting ammonia discharge permits without increasing the physical parameters of the treatment plant (aeration holding time, MLSS, sludge age, etc). Occasionally, there are other benefits from system use. For example, a WWTP may be able to maintain nitrification with lower sludge age or less aeration input when The ESI Nitrification System is in use.
The ESI Nitrification System can be used to:
Consistently Meet Nitrification Permit:
Some plants, due to inadequate plant design, excessive hydraulic or organic loading, or other uncontrollable factors, violate ammonia discharge limits on a regular basis. When the plant is typically coming close but not quite meeting permit, The ESI Nitrification System will most likely bring the plant within compliance. Ammonia levels that periodically exceed permit levels can be reduced.
Average ammonia levels can be reduced.
Delay or Avoid Plant Expansion: In some cases, a facility will consider expanding in order to consistently meet existing or pending nitrification limits. Often, The ESI Nitrification System for ammonia reduction can be used to meet permit until the expansion is complete. As long as the sole need is for improved nitrification, and not for a more difficult situation such as better SS or BOD removal, The ESI Nitrification System can potentially save a community the cost of unnecessary plant expansion.
Recover Rapidly from Upsets:
Without The ESI Nitrification System in use, a hydraulic or chemical shock can cause loss of nitrification that might last for weeks. With the ESI System in use, recovery occurs as rapidly as possible, often within a day or two rather than weeks.
Production of Nitrifying Bacteria
The ESI Nitrification System for ammonia reduction produces massive numbers of nitrifying bacteria. With use of the ESI Nitrification System, a treatment plant receives as many nitrifying bacteria in a single day as the treatment plant would normally get in 30 days from the raw influent wastewater.
As discussed previously, nitrifying bacteria have different requirements than heterotrophic bacteria do. Heterotrophic bacteria consume soluble organic carbon. And different heterotrophic bacteria not only grow in water with plenty of dissolved oxygen (D.O.), but with low D.O., or even anaerobic conditions (no D.O.).
The conditions in raw wastewater are excellent for growth of heterotrophic bacteria. There is an abundant organic food source, varying amounts of oxygen, and varying lengths of time available for growth and reproduction (depending on the residence time of sewage in the sewer pipes). Many technical specialists will state “all of the bacteria needed for successful biological wastewater treatment are present in the raw influent”. However, this is rarely the case with nitrifying bacteria.
The conditions required for growth of nitrifying bacteria are not present in sewer collection pipes (i.e., the collection system). Nitrifying bacteria are strictly aerobic, meaning that they require D.O. for growth and reproduction. The collection system rarely has significant level of D.O. Nitrifying bacteria do not grow well in the presence of high concentrations of soluble BOD. Of course, the soluble BOD level in the sewage collection system is quite high.
The concentration of heterotrophic bacteria in the collection system (raw influent wastewater) is quite high, generally in the range of 10’s of millions of bacteria per milliliter (ml). These bacteria are present in the waste that enters the collection system (in human waste, for example), plus heterotrophic bacteria grow and multiply easily in raw influent wastewater.
However, nitrifying bacteria DO NOT grow in the collection system environment. Plus, nitrifying bacteria are not among those bacteria that typically enter the sewage stream as part of human waste. Instead, the main source of nitrifying bacteria is in rain water that enters the collection system. As strictly aerobic bacteria, nitrifying bacteria live in the top few centimeters of topsoil. When it rains, some of the nitrifying bacteria are carried by rainwater into the sewage system.
The situation in the winter is more adverse. Again, the nitrifying bacteria live in the top few centimeters of topsoil. When the soil freezes, the nitrifying bacteria are immobilized, and do not enter the sewer system in sufficient quantities. Cold temperatures alone inhibit nitrification. But, the lack of nitrifying bacteria in raw sewage during the winter conditions makes cold weather nitrification even more challenging.
Due to many factors, the average concentration of nitrifying bacteria in raw influent wastewater is about 10 organisms per ml. Using this value, it is simple to compare the number of organisms fed to the treatment plant by raw influent wastewater compared to the number fed with the ESI Nitrification System.
The following calculation is based on a hypothetical 1 MGD (million gallons per day) treatment plant. With a concentration of 10 bacteria per ml, the number of nitrifying bacteria fed to the treatment plant in one day by the raw influent is 38 billion bacteria per day (10 nitrifying bacteria/ml times 1,000,000 gallons per day times 3,785 ml/gallon).
The ESI Nitrification System is designed to add as many nitrifying bacteria to the treatment plant as would normally be added in one month from the raw influent. The ESI Nitrification System is designed to add nitrifying bacteria at 30 times the rate contributed by raw influent. The reason is simple. Many WWTP’s do not have an ammonia permit in the winter period, but have an ammonia permit restriction in warmer months. Experience has shown that it takes an average of 30 days to make the transition from a non-nitrifying mode into permit-level nitrification. The ESI System offers a solution for speeding-up the process.
What is required for Using the ESI Nitrification System?
There are three basic parts to The ESI Nitrification System. First, the ESI Delivery System (MVD, LVD, etc), second a start-up liquid of concentrated nitrifying bacteria (EcoClear Concentrate) and, third, additional powder-form bacteria and nutrients (Powdered Ecobac) that are introduced to The ESI Delivery System on a weekly basis.
Initial Set-up Cost for the ESI Nitrification System
There will be an initial start up cost for the ESI Nitrification System that includes the Delivery System, liquid EcoClear Concentrate, and installation fees.
Ongoing Cost for the ESI Nitrification System
Weekly doses of Ecobac are required to keep the system operating. There may also be a cost to the end user for system maintenance by ESI service personnel.
Ready-To-Use EcoClear Concentrate (Instant Shake & Pour Solution)
Smaller treatment plants may not have the facilities to install or maintain a full ESI Delivery System. Or, the situation may be that there is only a periodic need to nitrification assistance, such as during an upset that may occur unexpectedly every few months. For these situations, a ready-to-use product can be kept on site so that it is ready to use as needed. This ready-to-use product does not require the use of a Delivery System.
The following are guidelines for applying ready-to-use product:
Plants with 6 to 12 hours retention time in secondary aeration:
Dose at the rate of 5 ppm for three days, 2 ppm for the next 4 days, followed by 1 ppm for the next seven days. Plant restart should be complete in 14 days or less. Example: Plant with 9 hours aeration detention time, and 1.6 MGD flow, requires 8 gallons per day for two days, 3.2 gallons per day for 4 days, and then 1.6 gallons per day for seven days. Total gallons required for this example is 40.1 gallons.
Plants with 12 to 36 or more hours of retention time in secondary aeration:
Dose at the rate of 3 ppm for 3 days, 1.5 ppm for the next 4 days, followed by 1 ppm for the next seven days. Plant restart should be complete in 14 days. Example: Plant with 24 hours of aeration detention time, 1.6 MGD flow, requires 4.8 gallons per day for 2 days, 2.4 gallons per day for 4 days, then 1.6 gallons per day for seven days. Total gallons required for this example is 30.4 gallons.
ESI System versus “Ready-To-Pour”
Which should you use?
You need to determine which solution provides the best value and benefits. The “Ready-To-Pour” is not as economical on a per day comparison, but also does not require the investment of a system, set-up, installation cost and ongoing operation. In addition, it can be used immediately upon receiving it. Please note that the shelf life of “Ready-To-Pour” EcoClear Concentrate is one year after receipt at your facility.
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