Case: At a small county wastewater treatment plant in Ohio the principal source of waste water was a soft drink bottling plant and the original design included both acid and base chemical addition directly at the plant using feedback control.
Observation: Accelerated chemical corrosion indicating the non-functioning of that the pH control scheme. The time delay in a feedback control system can become unstable resulting in over feeding chemical for a condition that has already occurred.
Solution: An equalization basin to be located at the soft drink company; Handling the pH swings by feeding a buffer introduced by gaseous feed of CO2 or solid feed of sodium bicarbonate.
José Enrique Martínez Cervera, Diseño plantas en SMAC Equipos Tratamientos Aguas, SLL, Valencia, Spain: The equalization basin is a good idea; select the correct volume and the stirrer power then install a neutralization chamber with a high speed stirrer and a pH control. With this system a lot of chemicals can be saved. It’s better than neutralize in the first tank.
In many cases an equalization basin is mandatory. Soft drink industry generates wastewater with large pH excursions resulting from CIP operations or sudden discharges of washing machines in plants that use glass bottles for the product. In some cases the use of CO2 is good, but in some others it is not. Designing an adequate system requires to know the total alkalinity of wastewater in order to assess its buffering capacity. pH data is not enough.
Agustinus Adi Anto, Technical Manager at Pt. Hara international, Indonesia: Neutralization by buffer is not a common thing to do at the wastewater treatment plan. Normally, we use HCl or NaOH for Neutralization at neutralization basin before it is introduced to the next process or in the equalizing tank to fasten up neutralization reaction.
Understanding the waste type, number of waste will help a lot to determine number of chemical and its type to fasten up neutralization. At normal condition, neutralization will generate sludge, which must be controlled to minimize its possibility to enter the equalisation tank.
Che Adnan Che Mat, Environmental Executive at Synthomer, Malaysia: To build the EQ tank the flow rate of that wastewater has to be considered. EQ tank needs to have five days retention time. If the effluent was in acidic sodium carbonate, the best chemical to increase the pH rather than NaOH.
Heath Edelman, PE, CHMM, Project Manager at Entech Engineering, Inc., Lancaster, Pennsylvania: Equalization is always a good idea. I use 20-minutes HRT at peak Q to get in the ballpark when designing EQ for pH control only. More may be required for temperature or other loading considerations for downstream processes. You can have too much EQ with easily degradable high organic waste streams, where too much stagnation can drive the pH down due to biological activity. You can burn up a lot of neutralization reactants as a result. If you have the ability to divert and collect high or low pH waste streams from cleaning, these often can be fed back into the process at a different time to cut down on neutralization chemical use.
Kris Hariharan, Head-Technical BacEnz, Chennai, India: While equalization tank helps in maintaining the pH, separating CIP alkaline water would help in handling of huge fluctuations co2 requirement. Similar technique is practiced in textile effluent by separating dyeing concentrate.
The presence of citric/sugar are good substrates for anaerobic digestion and methane production.
Terry Scheurman: I have done this several times. We sized the EQ tank to handle 24hrs of waste flow. This gave us sufficient time to respond. We installed recirculating pumps at 3x the influent flow. The discharge from the pumps went to the WWTP and the excess back to the influent to the EQ tank. We fed magnesium hydroxide slurry for pH control into the recirculating flow. This maintained a stable pH in the EQ tank and in the feed to the WWTP. The constant flow and stable chemistry kept the biomass in the WASP very healthy. In most bottling plants there will be sufficient micronutrients but you will want to do an assessment for trace minerals like iron, copper or selenium.
We handled some of this by feeding Ferric sulfate to the clarifier. This controlled Phosphorous discharge and maintained a low SS in the final plant effluent. The RAS would thus contain iron and handle that demand. Nearly all juice plants will have a low pH until or unless the plant does an alkaline clean-up of equipment. With a large EQ, the cleanings have little negative impact.
Your cost trade off is a smaller, EQ = more control issues and more chemicals used.
Bruno Peeters, water wizard-Antwerp, Belgium; CEO and Owner at Model Engineering: A well sized mixed EQ tank with at least six hours average HRT would be a key part of the solution. The six hours HRT in the EQ tank can be reduced depending on the CIP cycles and degree of pre-neutralization. The advantage of a mixed EQ tank with long HRT is that a large acid dosing pump (or better CO2 injection dosing) can be used with pulse width modulation PWM (0% – 100% variable duty as dosing time within fixed cycle time) thereby extending the dosing range by an additional 100:1 turndown ratio.
In addition, I would recommend to catch the large batches of spent caustic from CIP cleaning cycles in a separate buffer tank and bleed this small alkaline stream into the rest of the (more acid) wastewaters under modulating flow control governed by a master pH control at the outlet of the EQ tank. In this way effective auto-neutralisation can be achieved in the EQ tank on a daily basis with pH controlled acid dosing in stand-by as to handle the weekly cleaning cycles.
Jorge Edgardo López Hernández: There’s no point in using more than six hours HRT, particularly in cases where the wastewater coming from the soft drink facilities is going to be anaerobically treated. The reason is that in many cases this results in excessive acidification and higher acid alkaline reagents consumption due to AGVs neutralisation.
One more thing to consider is a dosing pump is going to be used, you have to take into account the turndown ratio of the pumps. Most dosing pumps have a 100:1 turndown ratio, so they cannot achieve proper pH control in those cases where there are large pH excursions, let’s say three or more order of magnitude and the wastewater exhibits a high buffering capacity.
An HRT=30 minutes in a neutralization tank might be very short regardless of the mixing intensity. That’s why an EQ tank is mandatory with auto-neutralization operation schemes.
Terry Scheurman, President at Applied Specialties, Inc., Cleveland/Akron, Ohio: Here is something to consider when you down size the EQ tank. Difficulty in control and costs both go higher with a smaller EQ tank.
At a Coca Cola and at a Pepsi plant, one in Ohio and one in Florida they were getting fined by their POTWs (to whom their final effluent went) for various violations of their permit.
The one plant used a tote of sulfuric acid every five days and two totes of caustic per week. Violations were in the range of $45K/month.
They installed a new EQ with 24hr detention, pumps and piping. It was approximately $300K. Once the new EQ was valved into service, the sulfuric dropped to zero lbs, the caustic dropped to zero lbs and we used one each 3000lb tote of magnesium hydroxide slurry every 10 days (~$1600).
The other plant was similar except the POTW was threatening to cut them off for P violations. There we added Ferric sulfate for $8K per year but still increased the EQ.
As a point of clarity, the POTW’s wanted to have assurity that the plants would have sufficient capacity to handle upsets in their operations and the 24hrs achieved that goal.
Bruno Peeters: In my 30 years of experience with water management and wastewater treatment in a range of beverage industries, it is more cost-effective to either separately collect the caustic CIP wastewater upstream (in the beverage plant) for subsequent bleeding in the mainly acid wastewater as already discussed and/or to automatically deviate the off-spec wastewater downstream (at the receiving end) to a calamity tank.
And even more interesting would be to convert the residual sugars and organic acids to methane (CH4) as to generate green energy i.o. paying charges and fines as to waste energy in the downstream public sewage treatment plant (POTW).
Jorge Edgardo López Hernández: Neutralisation goals may differ. In your particular case the facilities are discharging to a POTW. Then a large EQ tank seems to be adequate.
Bruno Peeters: A large EQ tank can compensate for the (lack of proper) manual/human controls. At least it gives the operator more time to react. The scheme with the smaller EQ tank and upstream pre-neutralization or calamity tank requires adequate automatic process controls as discussed. However Jorge Edgardo’s and my point about inserting a high rate anaerobic digestion with huge energy savings still holds especially with the aerobic treatment on site.
This discussion was initiated by Barry Walkenshaw, Project Manager, Caldwell, Ohio; Member of Collection Committee at Ohio Water Environment Association.