Is the presence of floating matter due to poorly treated FOG or injection of septic waste?
We operate a 25,650 PE/h treatment plant, low load (4.2 g/l), currently at 60% of its rated load, and we are encountering problems with floating solids and filamentous bacteria. We are wondering if this is a result of improperly treated fats (insufficient mixing in the hydrolysis tanks and lack of aeration in the biological reactor) or injection of sludge into the aeration basin from a rural treatment plant silo (1 m3/day, PH 7, COD between 15 and 25000 mg/l)?
It might be the result of both. You should avoid incorporating silo sludge into an aeration tank.
This sludge is septic, which encourages the growth of filamentous bacteria. The supernatant water from silos is often problematic. Would it be feasible to add this sludge directly to the dewatering process and see if the problems persist, which would tell you if it is related to the FOG treatment?
Start by looking at your mass load (food/mouths to feed).
Lack of oxygen or lack of food = filaments solution: extraction
If your C:N:P ratio is off or the supernatant contains volatile fatty acids, this will determine the type of bacteria, but that is not what's actually responsible for their growth.
Winter temperatures are favorable to the development of hydrophobic organic foam (high concentration in the aeration basin and diluted raw water). A microscopic analysis (with staining) should be able to identify the type of bacteria and the likely cause (Microthrix parvicella: fat, Thiothrix spp: nitrogen, etc.)
With a 60% load you should be around 2.5 g/l.
2.5 g/l in winter—that is borderline in terms of nitrification.
Your plant was conceived to process nitrogen in accordance with stipulated discharge levels.
In sizing to 4 g/l, sludge mass loading should be 0.07kg BOD5/kg volatile suspended solids (usually), and this at 100% of the load.
Mass loading and sludge age being related, if you respect that ratio, nitrification will be maintained (at 10-12 °C without exact TKN).
Then, the next procedure that would enable you to validate the sludge rate is to calculate volumetric loading in TNK, determine the speed of nitrification (and denitrification from soluble COD). Once these calculations are done, you confirm the required aerobic and anoxic treatment times. Based on that, you will know whether to increase the amount of volatile suspended solids.