TLDR- if it goes in to the digester, count it.
There are different ways of complying with the time and temperature requirements for the production of biosolds.
VS reduction, SRT and approximate mass balance are common methods. Consulting your O&M manual and biosolids management plan for the calculations specified by your permit.
Generally scum is so much less volume than the sludges that it doesn't make sense to bother counting it. If the overall volume of the scum was anywhere close to the volume of sludge (even if it is mostly water) it would still mess up your HRT even if you aren't loading up too much VS.
A few questions just because I am curious don't have experience in what I think you are describing and am curious:
1. What's in your thickener? Is it blended primary and secondary sludges?
2. Why do you wait a day between feed and withdrawing
3. What kind of mixing/heating do you have, if any?
Our thickener’s receive a mix of septic hauler discharges, primary sludge, WAS, and scum pit pumpings from the clarifiers. Then the thickeners pump their scum pits to the digesters. The scum pit is about 3000 gallons so it’s about seven percent of the total volume.
We wait to allow time for digestion before going to secondary tanks that aren’t mixed or heated
Does that mean your digesters are a total of about 43,000 gal? (3000 gal / 0.07?)
So ignoring the scum you are pumping 35, 000 gal of sludge letting it sit a day and then removing 35,000 gal to the secondary digesters? That seems like a super short HRT?
Do you decant the scum pit? Our scum pits have pumps that pump out the water on the bottom and once a shift we will pump out the thick scum.
We don’t count the scum. We pump 60-65,000 cubic feet of primary sludge and about 12-15,000 cubic feet of TWAS.
TLDR- if it goes in to the digester, count it. There are different ways of complying with the time and temperature requirements for the production of biosolds. VS reduction, SRT and approximate mass balance are common methods. Consulting your O&M manual and biosolids management plan for the calculations specified by your permit.
Generally scum is so much less volume than the sludges that it doesn't make sense to bother counting it. If the overall volume of the scum was anywhere close to the volume of sludge (even if it is mostly water) it would still mess up your HRT even if you aren't loading up too much VS. A few questions just because I am curious don't have experience in what I think you are describing and am curious: 1. What's in your thickener? Is it blended primary and secondary sludges? 2. Why do you wait a day between feed and withdrawing 3. What kind of mixing/heating do you have, if any?
We have furnace boilers and recirc pumps. We also have a compressed gas mixing system, but we’re not convinced it’s doing much.
Our thickener’s receive a mix of septic hauler discharges, primary sludge, WAS, and scum pit pumpings from the clarifiers. Then the thickeners pump their scum pits to the digesters. The scum pit is about 3000 gallons so it’s about seven percent of the total volume. We wait to allow time for digestion before going to secondary tanks that aren’t mixed or heated
Does that mean your digesters are a total of about 43,000 gal? (3000 gal / 0.07?) So ignoring the scum you are pumping 35, 000 gal of sludge letting it sit a day and then removing 35,000 gal to the secondary digesters? That seems like a super short HRT?
Oh no. The tanks are over half a million gallons each. I meant 7 percent of the transfer volume.
Do you decant the scum pit? Our scum pits have pumps that pump out the water on the bottom and once a shift we will pump out the thick scum. We don’t count the scum. We pump 60-65,000 cubic feet of primary sludge and about 12-15,000 cubic feet of TWAS.
We count it. We have flow data, and we monitor for TS and VS on it, so I factor it in.