The wastewater treatment plant was built 10 years ago, when the egg breaking capacity was at 1.5 million eggs per day and was engineered for handling an expansion up to 3 million eggs. Over 4 million eggs per day are currently being broken and the system is very over capacitated.
The key environmental problems include bad odor, mainly caused by rotten eggs (hydrogen sulfide), sludge removal and a COD value in the final water effluent that exceeds government standards.
PLANT WASTEWATER COLLECTING TANK:
- Capacity: 10.000 liter (no permission to build a bigger tank)
- pH: Varies between 2 and 13
- Influent 150 - 170 cbm of wastewater per day is pumped into this collecting tank; if there is a technical problem further down the wastewater treatment, then the wastewater or sludge can be routed into this collecting tank;
- COD: 5.000 up to peaks of 35.000 mg/L. Extremely high COD levels happen with accidental loads of egg products from the production.
Sulfuric acid is added permanently to neutralize the caustic (need 1.000 liter per 3 days).
The wastewater contains between one and two percent of the egg-load and all the chemicals used for cleaning and disinfecting the plant and the equipment.
Chemicals utilized that enter system:
- Caustic soda 2% (8.000 liter per week) is used for removing leftover egg whites and yolk from the system and egg products, which become solid from the walls of the pasteurization units.
- Nitric acid is used for cleaning the containers
- Hydrogen peroxide used for disinfections
- Detergents from the cleaning of the outside of the pasteurization units, the outside of the containers and the floors.
The high loss of eggs comes from the pasteurization units, where a blend of water and eggs is passing through and wasted until the correct drying ratio is reached for pasteurization.
SMALL AERATION BASIN:
Covered area (to hide the odor) with a capacity of 450 cbm;
One surface aerator:
- receives 150-170 cbm per day from the collector tank;
- wastewater flows into the aeration area through a hole in a separating wall;
- receives additionally wastewater - 35.000 liter per hour pump capacity - from the aeration area for premixing;
- pH is measured daily and should be in line with the pH of the aeration area, between 7.6 and 8.2;
- Capacity: 3200 cbm
- pH : 7.6 - 8.2
- Total Suspended Solids is 0.6 - 0.7 gram per liter, acceptable for the aerators;
Aeration tank constantly receives wastewater from the small aeration basin and sends wastewater into the small wastewater basin for premixing.
The average retention time of the wastewater in this aeration basin is 20 days. Three surface aerators suck up the wastewater from the bottom on to the surface and blow it into the air, taking up oxygen from the air.
During long-lasting warm periods, the upper layer might form a thick crust. If this breaks, the odor is intolerable.
At the beginning of the week, the minimum height of wastewater in this basin is 3,5 meters, which is the minimum height that the aerators can function. The maximum height is 5,0 meter and is reached by the end of the week. No wastewater is coming from the plant during the weekend)
9-10 cbm per hour of wastewater and sludge are discharged to the flotation unit.
Receives 9 - 10 cbm per hour of wastewater from the aeration area;
Can send 5 - 7 cbm per hour of water effluent to a pond;
Sends 3 - 4 cbm per hour of sludge to sludge storage,
Can also send the sludge to decanter sludge storage or back to the aeration area;
Flocculation is done by 'polyelectrolytes', which keep the sludge on the surface, on top of the wastewater (1/3 sludge, 2/3 wastewater).
A large amount of polymers is used for this flotation process. This results in the presence of still an important concentration of this polymer in the final treated wastewater that is going to the reed-pond.
Capacity is 700 cbm.
Aeration carpets are installed at the bottom of this basin.
Intake is from the flotation unit, at a rate of 3 - 4 cbm/hr.
Sludge has a solid content of about 3% (2.8 - 3.2)
This area has the most critical odor problems.
As long as the amount of sludge does not exceed half of the capacity of the tank, the 'aeration carpet' system is sufficient to feed the existing bacteriological system, which has been build up over the years in a natural way.
When however, the sludge exceeds half of the capacity of this storage basin, the aeration system cannot handle the sludge anymore.
In this case, there is not enough oxygen and after a certain time, a fast, dramatic turn-around takes place.
As a result, the brown color of the sludge becomes black (the bacteria are killed) and the rotten-egg odor permeates the whole neighborhood.
Note: It has happened in the past at some occasions, that in the other basins also the bacteria got killed and formed a black top layer, with the stinking odor as a result.
Sludge removal: presents also a serious problem. It can be pumped out for agriculture use, but this is illegal in wintertime (4-5 months per year).
The alternative is to send it to a decanter, after which the decanted sludge, with a solid content of 12% has to be taken away and burned, which is a very expensive solution.
To the heavy sludge storage, lime is added for drying and for odor protection.
POND + REED POND:
The clean effluent from the flotation unit floats into a pond of 225 cbm in total.
This includes a small reed-pond of 75 cbm and a checkpoint for controlling the COD, prior to the water flowing into a ditch. The COD should not exceed 125 mg/liter.
TREATMENT PLAN FOR THE WASTEWATER SYSTEM WITH AQUA CLEAN:
Desired results from the use of AquaClean are:
- Remove the source of hydrogen sulfide (H2S) completely and eliminate the odor.
- Convert the sulfide into elemental sulfur, and then to sulfate, which has no odor.
- Reduce the formation of sludge by 35 to 60 %.
- Clear pond of scum and reduce the COD below 125 mg per liter in the final effluent.
The basic function of a biological treatment system, such as AquaClean, is to convert soluble organics to carbon dioxide, water and bacterial cells. These cells can then be separated from the purified water and disposed of in a concentrated form called 'sludge'. Carbon dioxide gases off into the air, the water goes to the river and the cells remain in the sludge storage basin. AquaClean reduces the amount of sludge produced significantly, but not to zero, and improves the quality of sludge.
Apply the products when the in-flow of wastewater is at its lowest level (during the week-end).
For the first application, dilute 1 part AquaClean 5 parts of water and spray it.
For the first application in the sludge storage, it is highly recommended that the sludge storage does not contain more than 400 - 500 cbm sludge.
In case the basin has a solid floating layer of sludge, break that layer in three or four spots, so that the products when applied get down to the bottom of the basin.
Note: If during the treatment of Aqua Clean, there is an extra toxic load of egg-products due to an incident in the production, the system with is already equipped to handle this and will prevent system destabilization. It is however recommended that after such heavy loads, or after maintenance incidents at the wastewater purification area that an extra dosage of Aqua Clean be added.
An additional product called Aqua Clean OX (peroxide, but not hydrogen peroxide!) was added to the sludge storage. The OX is quite important in providing enough oxygen into the sludge storage, which AquaClean will need to do the best job. This OX should be added during minimum the first two months.
The use of OX is recommended when the sludge storage exceeds 50% capacity. This is because the air beds in the basin provide oxygen for only half the capacity of the basin.
To apply OX, it should be slurried in a ratio of 1 kg of OX to 3 - 4 liters of water prior to addition to the sludge storage.
RESULTS AND CONCLUSIONS
Based on the treatment plan, described here above, we started middle of May 2001. This was a time that the neighborhood was complaining about the very bad odor and the owner was threatened to close the plant.
With the use of AquaClean, the goals have been reached:
- No detectable odors
- Clear water
- COD of the effluent within specification.
It is important to pay permanent attention to the wastewater purification to be able to react to production or maintenance incidents and to control and adjust the treatment with AquaClean.