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On page 9 of the CoP it states : In nutrient-sensitive locations, the local authority should consider more stringent performance standards for nitrogen and phosphorus (Table 5.1), particularly where measures are needed to achieve the objectives of the Water Framework Directive.
Table 5.1 states Total Nitrogen as N =5mg/l Only for nutrient-sensitive locations
My question is fairly straight forward. Is it possible for any Domestic (up to PE10) Waste Water Treatment Unit that is on sale in Ireland AND has a European Cert (12566-3:2005) to meet the above standard of final effluent before being discharged to ground water or surface water.
I appreciate people may come on this forum and start talking about reed beds and peat bed and all sorts of beds BUT has anyone out there actually got proof that the Total N is CONSTANTLY meeting the 5mg/l requirement.
Quiet a few Site Assessors have been contacting me in recent weeks as Local Authorities are asking for the above standard.
I'll explain why I think its virtually impossible (in fact I don't think there is any WWTU manufacturer in Ireland) that can meet the 5mg/l.
Basically:
In sewage treatment terminology the various forms of nitrogen present are all reported as the nitrogen content of the compound, so making a nitrogen balance is easy. 1mg/l ammonia N = 1mg/l nitrate N = 1mg/l total nitrogen N
Nitrogen in the raw sewage is mainly in the form of ammonia and organic (amino acids and proteins). The oxidised forms of nitrogen, namely nitrate and nitrate of very low in raw sewage, so the total oxidised nitrogen is nearly always 0mg/l.
During primary settlement the organic nitrogen breaks down a bit and releases ammonia. The overall nitrogen in the sewage remains but as solids are settled out there is a reduction of about 4% across the settlement process.
In secondary treatment, oxygen is supplied to help the bacteria to eat the BOD and COD. Once the bacteria have consumed most of the BOD and COD then a different population of bacteria will consume the ammonia. This process needs lots of oxygen and the bacteria change the ammonia into nitrate and a little nitrite.
The effluent from a secondary treatment process can achieve quite low ammonia. However, the ammonia has been converted to nitrate/nitrite so the amount of nitrogen leaving the secondary treatment stage is still the same as came into the treatment stage.
In order to remove nitrogen from the process it is necessary to mix this nitrate rich effluent with a soluble BOD such as is present in the primary settlement tank. This enable some specialist bacteria to strip the oxygen off the nitrate/nitrite and release nitrogen gas from the process. This is the main route to achieve a low nitrate in the final effluent.
Total nitrogen is all the forms of nitrogen including the main 3 which are ammonia, nitrate and organic.
The final effluent will have some 20-30mg/l of suspended solids, and as these are mainly surplus bacteria they will have a nitrogen content of around 4% so this contributes 0.8 -1.2 mg/l total nitrogen.
The very good secondary treatment process will give 2-3mg/l ammonia which adds 2-3mg/l to the total nitrogen, so we now have 3-4mg/l with just the ammonia and the organic.
The nitrate level that can be achieved depends on the amount of recirculation, 100% recirculation gives 50% nitrate reduction potential, 200% recirculation gives 66.7% nitrate reduction and 300% gives 75%.
Over 300% recirculation tends to run into problems of too much oxygen being returned which prevents the special bacteria stripping the oxygen off the nitrate.
If we start with normal domestic sewage, based on 200 litres per person per day we have 50mg/l of total nitrogen coming into the plant. The primary tank removes about 4% of this nitrogen, so we have 48mg/l entering the secondary treatment stage. Here the bacteria oxidise the ammonia to a very good 2mg/l and produce surplus bacteria which find their way into the effluent taking about 1mg/l organic nitrogen with them. The remaining 47mg/l are now present as nitrate/nitrite. With 300% recycle this can be lowered to 11mg/l of nitrate,
So the final effluent will have a total nitrogen of 2+1+11=14mg/l
That is ammonia 2mg/l as Nitrogen
Organic nitrogen 1mg/l as Nitrogen
Nitrate/nitrite 11mg/l as Nitrogen
Total Nitrogen 14mg/l N |
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