We operate an industrial aerated lagoon. Recent tightening of discharge standards (down to 10 mg/l TN) requires implementing a more advanced nitrogen treatment system. Our industrial client is looking for an affordable solution. One idea is to turn the aerated lagoon into an SBR-type activated sludge facility (we are well aware of the restrictions in terms of volume and mass loading). Does anyone have any experience with this type of modification?
Thank you in advance
A number of paper mills converted early aerated lagoons into activated sludge systems. The base lagoon served as the biological reactor with the addition of high speed surface aerators to provide the required energy for biomass suspension. Most of these systems operate at lower MLSS than typical industrial activated sludge with MLSS in the 1,000 - 2,000 mg/L range. Care must be taken in adding mixers/aerators to avoid breaking the clay liner in most of the older systems.
An SBR approach is interesting but you may have trouble with re-suspending solids after settling as they will accumulate in the corners and in sludge islands.
We have experience in this area; we have implemented this modification in several of our industrial treatment plants with decent success rates...
For starters, the sludge obviously has to have good settleability. The other problem is how to manage the nitrogen. In almost all cases, our effluent has low levels of ammoniacal nitrogen requiring minimal nitrification, which makes the whole process a lot easier.
On the other hand, where levels of nitrogen compounds are high, you would have to perform denitrification before the settling phase. We haven't had to deal with that yet but the day will come.
Our last experiment was undertaken with no guarantee in terms of results and was based on a very unbalanced effluent, ratio of COD/N < 5; the customer did not want to implement the system we suggested. Result: a nitrite stock that stalled the biological process...
Generally we do 2 or even 4 cycles a day. Settling time varies from 60 to 120 min. The risk in my opinion is having extended aeration times that foster the development of filamentous material.
Consider installing a small buffer so as to prevent feeding the effluent in the SBR during settling and extraction (unfortunately that is not possible in our operation...).
Size the SBR so as not to require too high an SS concentration, which necessarily encourages settling.
Watch out for fats that promote scum, filaments and light sludge that doesn't settle as well.
SS at output of less than 30 mg/l is often difficult to maintain, depending on the sludge index, fat content, SS in the SBR.
This configuration is indeed attractive because it's easy to implement, but in terms of calculations, you have to factor in a safety cushion that at the same time won't leave you with a mass load of less than 0.07 kg COD/kg DM. Aeration capacity must provide some room to maneuver.
I have no feedback on a conversion from aerated lagoon to SBR.
But an SBR requires the following dimensions for height:
A water level range > 1 and 3 m, which is the minimum amount of biomass needed for the biological treatment of pollutants.
And for effective nitrogen treatment, a minimum of 2-3 SBR units.
The first item to check: is the height of your aerated lagoon 4 meters or more?
If not, you could use steel tanks (if the effluent is not corrosive) for the SBR stages; these are not very expensive.
Thank you for your reply. We've hit the nub of the issue here because the height of the lagoon is 2.4 m. But since a lagoon involves a sizeable surface area, it actually enables us to manage input volume and the required amount of biomass. However assuming an adverse sludge volume index (agri-food effluent), I don't know how we’d manage settling and discharge of clear water.
I think that with a height of only 2.4 meters and an unfavorable sludge index... the clarifying phase may be tricky: at the end of the settling phase you would need a sludge bed of max 1 m in height, and you'd have to ensure a good water level for proper clarification at 1.4 m.
In terms of control and instrumentation, that can be done by continuous monitoring of the sludge blanket via a turbidity sensor that measures the suspended solids at clarified water output.
NB: on an SBR, the index is 25 % > than in a standard activated sludge process.
To manage discharge of the clarified water, you can use a floating weir:
Regarding aeration, because of the reduced height you would need floating surface turbines/aerators.