Sizing of an equalization basin 1

[umrcheme]

We are working on an equalization basin for a 0.35 MGD WWTP. I have come up with the theoretical value, (40,000 gal) but all the literature I have read says to increase that number by 10-40%. How does one decide on the appropriate factor? Additionally, at what point should aeration of the basin be considered?

 

[RWF7437]

The "appropriate" number depends on how confident you are of the input data; especially flows and durations. If those numbers are based on a long record of flows you can be more confident. If they are based on a shorter, or no record then play safe and upsize the basin. Also remember that future flows can ONLY be estimates

Wastewater goes septic in three to four hours. If your expected detention time in the basin is more than that consider adding aeration. By all means, design the basin so that aeration can be easily added later if needed.

Good luck

 

[umrcheme]

Thanks for the information. Are there any design criteria with regards to an equalization basin? I am interested in bottom configuration and baffling. I have been reading Metcalf and Eddy. Are there any other aspects one should be concerned with? Additonally, we are considering an above ground tank do to the small amt of land available and high water table. Is this an acceptable approach?

 

[semo]

As RWF stated, your sizing should be based somewhat on your comfort level with your excessive flows. If you undersize it you might exceed its capacity and cause an SSO. If you oversize it you waste some construction cost and possible operating costs (excessive aeration/mixing).

Depending on how long you hold the waste and it's source (I&I, surges from industrial users, etc.) you might or might not need to aerate it.

Tanks have been used for storage. What will the baffling do? If it is for aeration, mixing, or other primary treatment ok, if not, do you really need it? Make the bottom where the tank will drain. If it doesn't, you will have odors. Make sure your hydraulics work.

One of my main concerns would be the regulating authority. What will they allow?

 

[umrcheme]

Thanks for the input. Our regulating authority does not directly address flow equalization or tankage. Therefore, we defer to the 10-state standards.

The WWTP currently operates with two headworks pumps which do not have vfds. Therefore they are pushing full flow or no flow into the facility. They also have very little storage upstream. Due to the configuration, the plant has been subject to washouts and will be under an agreed order with IDEM (Indiana EPA) shortly to correct the problems. We are proposing to install an equalization basin/tank upstream of the headworks pumps. Additionally, the headworks pumps would be reconfigured to operate off of vfds.

Since the equalization basin would be inline, it would always have wastewater in it but it would turn over frequently. The only purpose I would see for baffling would be for mixing and/or aeration.

 

[ema]

Depending on your wastewater characteristics (SS), it would be better to maintain a minimum velocity of 0,3 m/s in your tank. You might consider using baffles for this purpose; if not appropriate try installing submersible mixers. But for a small tank of 40,000 gal, a submersible mixer would be a better alternative for mixing.

Need for aeration depends much more on the kind of process that you will use for the treatment.