DAP (diammonium phoshate) should be about 20% N and 20%P. If you don't need the phosphorous you won't want to add extra because you'll probably pay for removing it chemically on the backend.

Urea (somewhere around 40-50% nitrogen) is a good source. Enzymes are needed to break this down to ammonia, but these should be present in your MLSS to begin with. Typically the only time there's problems with urea is if the OUR (oxygen uptake rate) is very high and the urea can't be broken down by the time the bacteria need it as a nutrient.

Before doing any nutrient addition I would do the following: Take a sample of your MLSS right before the settling cycle, filter it, and test it for ammonia, nitrate, and nitrtire. Add the three up and if it's around 1 mg/L there's plenty of nitrogen available. (You want to see a small residual left over). For phosphorous you'll want to make sure you have >0.5 or so orthophoshate mg/L in the filtered MLSS.

Also a reverse india ink stain is recommended to judge the "slime" levels in the sludge. Elevated EPS (slime) can be indicative of nutrient deficient conditions. Of note- the 100:5:1 ratio is only a starting point- typically the maximum amount needed. Each plant's actual nutrient demands are different and less may be needed. The biggest factors here are with increased temperature and as the sludge age gets longer more nutrients are recycled throughout the process and bacteria die and less are needed. Also note that sometimes the required ratio may be different seasonally, as in the winter there's less endogeny due to slower metabolism of the bacteria-therefore less nutrient recyced and a higher nutrient demand.

hope this helps,

Ryan Hennessy

I'd consider anything over 55 mg/L/hr (the speed limit) high for OUR. Orthophoshate should be tested in the filtered residual because this is phosphorous in a soluble form which is readily available to the bacteria as a nutrient.

The bacteria will produce slime due to a shunt metabolism or unbalanced growth when they don't have the nutrients available to produce N and P containing cell material.

OUR can be a factor because the nutrients have to be in a form for the bacteria to take up. (ammonia, nitrate, orthophoshate) If the OUR is high, the bugs are growing quickly and need the nutrients readily available. In the case of urea if the enzymes haven't broken it down into ammonia, it's not available at the time. Sometimes in these cases nutrients need to be dosed on the incoming orthophoshate and ammonia. This is rare though.

What type of wastewater are you treating?

Thank you very much for answering my inquiries, we are treating wastewater from a yogurt factory, do you have any experiences with this type of wastewater? normally what problems should we expect when treating yogurt wastewater?