The WWTP itself is located in the East Midlands, and treats dairy waste from a large processing plant, treating c1,500m3 of effluent per day.
On 30th July the ammonia (NH3) levels in the permeate from the MBR at the plant began to increase. The effluent plant discharges to a river and as such, the consent limit is stipulated by the Environment Agency at 4mg/l. Within three days, the ammonia levels had exceeded this limit and due to the large volumes of effluent treated on this plant, the situation threatened to close the production facility as it would have been financially unviable to tanker all the effluent away.
This report aims to discuss the possible causes of this increase in ammonia levels, the effect this had on the plant, the actions taken by the plant operator to rectify the situation, and the procedure that should be followed should this incident reoccur in the future.
The main function of the aerated lagoon is to biologically degrade COD/BOD in the waste water from the factory below the agreed consent limit of 15mg/l. However, the wastewater from the factory contains around 15-20mg/l of ammonical nitrogen and the discharge consent limit is 4mg/l, therefore the secondary function that must be carried out by the lagoon is the reduction of ammonia content through Nitrification.
Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. The oxidation of ammonia into nitrite, and the subsequent oxidation to nitrate are performed by two different (nitrifying) bacteria. The first step is carried out by bacteria known as Nitrosomonas and the second step (oxidation of nitrite into nitrate) is done by bacteria called Nitrobacter.
The rates of nitrification reaction are highly dependent on a number of environmental factors. These include the substrate (COD) and oxygen concentration, temperature, pH, and the presence of toxic or inhibiting substances; many substances can potentially inhibit the nitrification reactions and metals are particularly strong inhibitors. When exposed to more than one inhibitor, the extent of inhibition increases greatly.
Nitrifying bacteria are especially sensitive to low oxygen and high or low COD concentrations and as such the DO level should always be maintained above 2mg/l with an F/M ratio of 0.05-0.15. pH also has a strong effect on nitrification rates and the optimum range is between 7 and 8. They are less sensitive to temperature although it is advised that this be maintained at 25-30°C.
- On Friday 27th July a new aeration blower was switched on in the lagoon.
- On Monday 30th July the ammonia levels in the MBR permeate had risen from the usual levels of 0.01-0.05mg/l to 1.98mg/l which raised concern with the ETP operators.
- By Tuesday 31st July this level had almost doubled to 3.78mg/l.
- Wednesday 1st August the ammonia level had reached 5.9mg/l, exceeding the consent limit and the plant had been put into recirculation.
- On Thursday 2nd August it was decided that there was a definite lack of Nitrifying bacteria in the lagoon as the ammonia levels had continued to rise and had reached 9mg/l. Enquiries were made as to methods of reseeding the lagoon with nitrifyers to reduce ammonia levels and discussed with Cleveland Biotech Ltd.
- An order number was raised on Friday 3rd August for 10kg of concentrated Nitrifying bacteria from Cleveland Biotech Ltd and it was couriered to site the same day. The ammonia levels had reached a plateau of 9-11mg/l and 5kg of the nitrifying bacteria were added to the lagoon once it arrived on Friday evening.
- A further 2.5kg of bacteria were added to the lagoon on Saturday and Sunday as instructed by Cleveland Biotech and it was suggested that a significant difference should be noticed within 4-5 days, this period takes into account the ‘doubling time’ of the nitrifying bacteria in order to reach the required critical mass.
- To keep the lagoon level down while the ammonia levels were high, a scheme was set up by the ETP operators where the permeate was mixed with the treated effluent from the “Old Plant” as this was still very low in ammonia. This resulted in a final discharge to river below the consent limit of 4mg/l although this was monitored every 2 hours to ensure the limit was adhered to.
- The ammonia levels in the MBR permeate remained at 9-11mg/l until Wednesday (8th) morning at which point it dropped to 8.3mg/l.
- On Thursday 9th August the nitrifying bacteria had reached “critical mass” and the ammonia level was back within consent from the MBR. By Friday there was almost no ammonia with a reading of just 0.04mg/l (as it was before the incident occurred).
Incident Cause(s) Discussion
As discussed above, there are many factors that should be controlled in order to enable nitrification in the lagoon. If the conditions are not correct, nitrification can be inhibited and as nitrosomonas and nitrobacter have a lengthy doubling time of c.24 hours it can take a long time to recover.
When nitrification begins to fail in an activated sludge plant, tests should be carried out for any inhibitors in the wastewater from the production facility such as metals and other toxic substances. In this case however, it is doubtful that this was a likely cause of the ammonia increase as the sister plant was unaffected and still managed to reduce the ammonia levels despite the fact that it receives exactly the same waste stream as the MBR. The ammonia levels in the crude effluent stream are also monitored and there were no noticeable increases indicating that ammonia was introduced at a later stage in the plant.
As well as toxicity in the waste, there are other factors described above such as low oxygen, high/low pH or temperature and high/low F/M ratio. There were no significant differences in any of these parameters that could have had such an impact to raise ammonia levels as they were. The dissolved oxygen levels in the days and weeks were mostly above or close to 2mg/l, the pH did read slightly high on occasion but not to the extent that could have prevented nitrification, the temperature of the lagoon does not fluctuate by much and is generally 23-25°C which is adequate for nitrification.
The only inhibiting factor that is evident from the data is the very low F/M ratios; the lagoon has been under-loaded for some time at an F/M ratio of around 0.018 and the desired ratio for nitrification to occur is 0.05-0.15.However, the F/M ratio has been low for many months and nitrification has still occurred as the ammonia levels had always been low; this does not therefore account for the rapid increase in ammonia levels experienced at the end of July.
On inspection of the lagoon and the aeration distribution within it, it became apparent that there are large areas which could be described as “Dead Spots” where there is little or no aeration causing anaerobic conditions. In the absence of oxygen, nitrification cannot occur so there would be areas where no nitrifying bacteria could function or reproduce therefore limiting the capacity of ammonia consumption. Also, these anoxic zones can cause adverse conditions where the bacteria in these areas actually die and subsequently release large amounts of ammonia; if mixing from aeration is limited, these areas can stay anaerobic for sustained periods of time with little overall effect. However, if these “Dead Zones” are somehow mixed with the rest of the lagoon contents the ammonia released by the dead bacteria could then be carried through in the permeate.
The most likely theory as to how this incident occurred is that some of these anaerobic areas were holding large amounts of ammonia and were then mixed by the new blower when it was initially switched due to a surge of pressure. As the conditions for nitrification in terms of F/M ratio had been limited for a long time, there were not enough nitrifyers present to deal with the rapid increase in ammonia resulting in a carry through to the permeate. The addition of the concentrated Nitrifying bacteria solved this problem within the time scale suggested by Cleveland Biotech therefore confirming that there were not really any inhibiting factors for Nitrification other than the lack of Nitrifyers in the first place. It really seems no coincidence that these issues directly followed the commissioning of the new blower when no other parameters had seen any significant changes that could account for the ammonia increase.
There are a number of factors that have been mentioned in this report that require attention and monitoring in the future to reduce the risk of this incident reoccurring. Obviously, the parameters discussed need to be maintained to ensure the conditions are correct for nitrification to occur:
- Dissolved oxygen needs to be =2mg/l in all areas of the lagoon therefore any anaerobic zones should be addressed and eradicated i.e. divers should be employed if necessary to unblock any aeration injectors.
- Temperature should be continuously monitored in the lagoon to ensure that it is in the region of 25-30°C not only to maintain nitrification but to ensure COD removal is efficient.
- pH must be maintained between the optimum conditions of pH 7-8. If this proves difficult, a pH correction system should be considered with continuous (accurate) monitoring as it is only monitored using a hand held probe at present.
- The F/M ratio should be increased to meet the minimum criteria of 0.05. This obviously requires either more COD or less MLSS; as the only parameter that can be easily controlled is MLSS this should be slowly reduced over the coming weeks/months to rectify the situation and further promote the growth of nitrifyers.
All the mentioned measures above are preventative to try and ensure that the incident cannot occur again. However, should there be another incident where ammonia levels increase past the consent criteria, there should be a ‘reactive’ plan in place as the implications of a halt in discharge to river are extensive in that it could ultimately result in the plant production being stopped. A reactive plan would ensure that the levels are reduced in the shortest possible time hopefully negating the requirement to tanker out or stop production.
The method used to rectify this particular incident, i.e. contacting Cleveland Biotech, worked very well and should be employed again should this incident resurface in the future. However, it would be advisable to add Cleveland Biotech’s nitrifying bacteria at an earlier stage so they can have an effect in a timelier manner. As the ammonia levels are usually so low at around 0.04mg/l, an increase to above 1mg/l would suggest a problem although this is still below consent. If the bacteria were added at this stage of an incident, it is possible that the consent may not be exceeded and if it is, just for a day or so which could be easily managed. The procedure of mixing the MBR permeate with the final effluent of the Old Plant also worked very well and again should be employed in the future if this incident reoccurs.
Cleveland Biotech offer a service where they guarantee that they would have the required amount of Nitrifying bacteria for the lagoon by having it stored under refrigerated conditions ready for dispatch the same day if required. There is a rental charge to be paid for this but it may be worth considering for peace of mind in the near future especially as the employment of divers does have the potential to mix up the current “dead zones” in the lagoon.
Although it is quite difficult to determine the exact cause of this incident, the evidence shown by the data suggests that there were no significant changes in the effluent from the plant or the normal conditions (pH, Temp, F/M) of the lagoon that could account for the increase in ammonia. Therefore, it is fair to assume that the commissioning of the new blower and the mixing of the anaerobic zones was the major trigger for the rapid increase and subsequent consent breach in ammonia levels as it is known that dead bacteria release ammonia. From the data, it is apparent that the conditions were not ideal for nitrification to occur as the F/M ratio has been low for a sustained period of time. It was however coping under normal conditions but when there was a large release of extra ammonia, there were not enough nitrifyers present to deal with the increase. The addition of Cleveland Biotech’s nitrifying bacteria solved the issue within days therefore confirming the fact that there were no toxic inhibitors in the waste stream and that it was more a lack of nitrifyers than a sudden killing off of the correct bacteria.
In the future, the conditions should be altered and monitored to accommodate nitrification more easily and the anaerobic areas of the lagoon should be eliminated to ensure that large concentrations of ammonia can no longer be accumulated.