Based on a technical and economic analysis of the situation, Durpro proposed a multistage water treatment system that would allow for the recovery MBR (Membrane Bioreactor) permeate for use in the cooling tunnel.
The treatment process
Approximately 30 000 litres of water is drawn from the wells daily. This water first goes through a reverse osmosis (RO) system to remove minerals and dissolved solids. Before being used, the water is filtered further and disinfected using ultraviolet (UV) irradiation.
This water is used for production, cleaning, boilers, and the cooling tunnel. When used for cleaning, the water is then sent to a second treatment cycle. This is now wastewater.
The second treatment cycle starts with a primary filtration to remove the largest contaminants. The wastewater transfers to several equalization tanks. It is then received by two membrane bioreactors (MBR).
After leaving the MBR, the water first passes through a double softener. Then passes through a nanofiltration system and two reverse osmosis systems. The wastewater rejected by the nanofiltration stage (20% of the influent) is returned to the MBR. The first reverse osmosis system rejects 15% to the septic system. The RO system recovers 90%, recirculating its rejection back into the system.
On average, Crofter's produces 12,000 litres of wastewater per day. Of the 12,000 litres, approximately 8,000 are reused for cleaning and in the pasteurizing cooling tunnel. Less than 900 litres per day are rejected to the septic system.
Contaminant concentrations
Crofter's
private water treatment plant is not only impressive for its wastewater
recovery and reuse capabilities. The concentrations
it faces are abnormally
high, yet the treatment is done consistently and efficiently.
Although membrane bioreactors are exceptionally effective against MLSS, they still have maximum recommended concentrations for their treatment. For effective treatment of MLSS by Membrane Bioreactors, the maximum recommended concentrations are generally around 9000 ppm. At Crofter's, MLSS concentrations reach record highs of 25,000ppm and higher.
Despite these high concentrations, the treatment is efficient because the concentrations are very constant and are mainly caused by natural sugar carbohydrates. Due to the consistency and quality of the effluent to be treated, the bacteria in the sludge have become accustomed to such an environment and have the ability to treat the water despite the high levels of contamination.
Regarding COD, Crofter's deals with relatively normal concentrations of +/- 50 mg/L. The volume of the aeration tank allows the company to treat these chemical oxygen demands without any problems.
Finally, the conductivity downstream of the membrane bioreactors is relatively high. With concentrations around 700 uS/cm, the nanofiltration step and the two reverse osmosis steps are not excessive since, after these steps, the water has an average conductivity of 5 uS/cm.
