Nitrification

Nitrification Intimidation
Part One

by David Buffum

As regulatory agencies increasingly tighten discharge permits in an attempt to clean up the nations waterways, it stands to reason that ammonia levels be targeted. Ammonia, present in most wastewaters both domestic and industrial, has been shown to be toxic at relatively low levels when introduced to the receiving stream, especially during the summer months when dilution factors are at their lowest. Fortunately, there is no magic involved in the biological process of wastewater treatment. Toxic ammonia (NH3) can be reduced through the process of nitrification to its non-toxic form, nitrate (NO3), before being discharged. However, since there is reason to believe that a discharge high in nitrate may in itself be undesirable, the process of nitrification can be easily extended to include de-nitrification, where the problematic nitrogen is ultimately converted to nitrogen gas (N2) and returned to the atmosphere from where it originally came.

The nitrifying bacterium Nitrosomonas is of primary importance for the oxidation of ammonia to nitrite (NO2). Nitrobacter, in turn oxidizes the nitrite ion (NO2), which is toxic to plants, to nitrate (NO3), in which form it is absorbed by plants and directly available to them. ^[1] While this single stage, the process of nitrification, is reliant upon two specific, slow growing, aerobic bacteria, several genera of bacteria are capable of carrying out the final stage or the process of de-nitrification under anaerobic conditions by converting nitrate (NO3) to nitrogen gas (N2). Many of these de-nitrifying bacteria are facultative anaerobes, organisms that may function either as anaerobes or as aerobes. This becomes an important consideration to the wastewater treatment plant operator or anyone involved in the design of nitrification facilities due to the fact that it is not possible, under normal conditions, to grow sufficient quantities of aerobic nitrifying bacteria without also cultivating a variety of de-nitrifying bacteria. These facultative de-nitrifying organisms prefer to utilize dissolved oxygen for necessary life functions but can and will utilize the chemically bound oxygen contained in the nitrate (NO3) molecule if forced to do so by introduction into an anaerobic environment. Of the total pounds of mixed liquor under aeration at any given time, only a small percentage will make up the nitrifier faction, making it necessary to increase the mixed liquor concentration, depending upon influent ammonia levels, to assure an adequate nitrifier population is available. It should be anticipated that a nitrified mixed liquor, teaming with microorganisms and nitrates (NO3) as it enters the secondary clarifiers, will quickly consume the available dissolved oxygen in the wastestream. As the dissolved oxygen levels approach zero in the sludge blanket, the facultative organisms will begin the de-nitrification process and release nitrogen gas in the clarifiers; the proper and desired reaction, but in the wrong place. The release of fine nitrogen bubbles in the secondary clarifiers can be associated with poor sludge settleability and the loss of solids from the clarifiers. Vastly increasing return sludge rates can help control the amount of de-nitrification in the clarifiers but is rarely adequate to totally control the problem. This also defeats the purpose of the clarifiers where it is desirable to concentrate and thicken the sludge before wasting. De-nitrification is an inevitable consequence of nitrification. Any up-grade to nitrification should include provisions for de-nitrification, since it is clear that the best solution for preventing clarifier de-nitrification problems is to minimize the amount of nitrate entering the clarifiers.

Although the toxicity issue associated with the discharge of ammonia has only been stressed by regulatory agencies within the State of Maine within the last decade, as evidenced by the increased number of training sessions devoted to nitrification and, to a lesser degree, de-nitrification; both biological processes are naturally occurring and have been around for a long time. This is not rocket science and certainly within the grasp of any proficient operator with a good understanding of the principals of wastewater treatment and the nitrogen cycle. Numerous reference materials are available on the subject to answer any questions an operator may have. With minimal research it becomes clear that, if the proper environmental conditions are met, most biological systems are capable of achieving nitrification and de-nitrification. There are a number of factors associated with the make-up of the wastestream; temperature, alkalinity, influent ammonia levels, consistency of loadings, etc., that will greatly influence the process and can not be ignored but two are crucial; time and air, as the following case history attempts to reveal.

A certain Sewer District treats a combination of domestic and industrial (tannery) waste. The original facility was designed for a flow of 0.6 MGD, half tannery and half domestic, but the industry exceeded their allotted capacity from day one. Treatment was never spectacular and the plant routinely violated permit limits. Despite this, permitted flow levels were increased to 0.9 MGD and then to 1.1 MGD; in both cases without any increase in tankage at the facility. In the early eighties, it was discovered that the introduction of an aluminum compound prior to discharge by the tannery could greatly improve upon primary clarifier performance. This made it possible to remove most of the settleable and floatable solids entering the plant, in excess of ninety percent of the incoming load, in the primary clarifiers. This was vital to the operation of the facility since the original aeration system could barely handle the burden imposed upon it, even after improvements in primary treatment. It was a common occurrence to have aeration system dissolved oxygen levels drop below one during the summer months. The plant did begin to meet discharge limits but with two exceptions; disinfection and toxicity.

Disinfection issues were complicated by fluctuating loads and a highly colored discharge but the problems were mainly due to lack of contact chamber detention time, as one might expect since the system designed for a flow of 0.6 MGD was now routinely seeing flows close to 1.1 MGD. On an average production day, influent ammonia levels were frequently in excess of 100 mg/l with effluent levels as high as 65mg/l. Clearly, although a small amount of ammonia was lost or removed from the system, most of it exited the plant, as ammonia (NH3) with the final effluent. Somewhat surprisingly, routine analysis of the plants discharge over the years never revealed anything exceptional, other than slightly elevated trivalent chromium levels and high ammonia levels. The chronic effluent toxicity problems could logically be attributed to effluent ammonia levels.

In the early nineties, the District was issued an EPA Administrative Order forcing it to deal with its ongoing disinfection and toxicity problems. A local engineering firm which had already worked on a number of projects for the District and was familiar with the facility was contracted. Although high effluent ammonia levels routinely existed, the District was compelled to appropriate the funding necessary for a full Toxicity Identification and Reduction Evaluation. The testing quickly confirmed that ammonia was the primary toxicant in the plants effluent. While all this was taking place, the industry was automating its mills and making additional in house changes to reduce water consumption and thus water discharged to the public sewer. Additionally, like many factories, the tannery shuts down for a two week period each summer. An interesting phenominom began to occur each year towards the end of the tannerys shut down period; chlorine residuals would drop despite increasing chlorine dosages being applied to the chlorine contact chamber. The lack of flow during shut down greatly increased the plants overall detention time and nitrification would begin. The process was not complete however, and the plants ammonia was not able to make the full conversion to nitrate. The ammonia was being converted to nitrite (NO2), the intermediate step between ammonia and nitrate. Nitrite is known for creating havoc with chlorination.

The Districts’ engineers developed plans for up-grading the facility to achieve “single stage nitrification” and the plant was up-graded by July of 1997. Issues surrounding de-nitrification were never specifically addressed by the engineers and were not included in their contract with the District; presumably, because they did not thoroughly understand the affect de-nitrification would have on the overall treatment process.

A new primary and secondary clarifier was constructed and the chlorine contact chamber doubled in size. The main feature, for purposes of nitrification, was an increase in aeration capacity. The original mechanical system was replaced with three centrifugal blowers capable of supplying air to a fine bubble floor mounted diffuser arrangement. Tank space was increased from two tanks totaling 0.316 MG to three tanks with a combined capacity of 0.962 MG. This satisfied the two crucial requirements necessary for nitrification vastly increasing the amount of air able to be supplied to the aeration tanks as well as increasing the detention time through the tanks. Nitrification established itself within several weeks of placing the up-graded system on line.

As nitrification continued, small bubbles could be observed rising from the plants secondary clarifiers. The plants secondary sludge had never settled very well but, up until the plant up-grade, had been controllable. The fine (nitrogen) bubbles in the secondary clarifiers continually carried small sludge particals from the clarifiers while sludge blankets began rising; soon reaching a point where the sludge would wash out of the clarifiers. Although the original plant, before nitrification, had never experienced problems meeting effluent settleable solids regulations, effluent settleable solids levels became a consistent source of permit violations. Return pumping was increased to maximum but the problem continued. It was also impossible to raise the mixed liquor inventory to the desired levels.

The plant continued to nitrify turning virtually all the incoming ammonia to nitrate and for the first time in the history of the District, bioassays tested satisfactorily. EPA had given the District a one year ‘grace period’ knowing that most any new or up-graded system would require a certain amount of time to work the bugs out. The engineers were reluctant to acknowledge the fact that de-nitrification was occurring and creating operational problems, preferring to focus on the success of the up-grade in eliminating the effluent toxicity problem while continuing to regard “single-stage nitrification” as a viable process.

Editors Note: This article will be continued in the next issue of NEWWN.