Chicken processing plants use large quantities of clean water in processing and production. This means there is a very high risk of environmental pollution, as more than 90percent of clean water is converted into wastewater. Ricardo Camacho and Luis Huerta report.
Water has four basic functions in most chicken processing plants (CPPs): to clean the animal channels during the process and afterwards; to cool down the channels after taking out the internals (guts); to clean the installation in general.
Wastewater generated at the CPP has a high concentration of colloidal suspended solids (blood) and waste (mainly feathers, excrement and pieces of chicken), BOD5, grease and oils, and though it might sound strange, colouring. The last component is due to the fact that in Mexico commercial chickens are coloured to have a yellowish appearance. Otherwise consumers would not buy the chicken. The average concentrations of the above components are as follows:
In some plants the above values are even higher since not all the plants have modern equipment that allows the waste (guts, excrement, blood and feathers) to be processed without introducing water to the cleaning process. This obviously means that expectations for efficiency at the wastewater treatment plant (WWTP) are too high, as we should remember that the BOD5 discharge limit for treated effluent generally must be under 200mg/l.
Following on from what has been mentioned earlier, the wastewater treatment process always starts with the processing of the chicken. It is clear that the water can only be treated adequately when it has not been polluted more than is necessary.
The best way of treating wastewater from the CPP requires that it will be treated as soon as possible in order to prevent the dissolving of pollutants that can be removed in a physical/chemical treatment process.
The first part of the treatment consists of screening to separate coarser materials, during which most guts and feathers are removed (most modern CPPs separate the feathers, which contain effluents). This separation stage is normally achieved using equipment supplied by the same company that supplies the chicken processing equipment, and sometimes this separation equipment is not considered as part of the wastewater treatment process. However, this separation has a fundamental influence on the functioning of all the WWTP.
The next step consists of filtering the water through a curved or drum screen with a fine slot width with openings of some 0.50mm to separate all larger particles, and hence clean the effluent for the next treatment step.
Once the water has been screened it can be treated by physical/chemical processes or by biological treatment. According to our experience the chemical treatment process is the most suitable because it requires less space, is easier to operate and is less sensitive. However, the best alternative we have seen is a combination of physical, chemical and biological processes.
For CPP effluent the best solution is a chemical process, which consists of: flocculation pipe and a flotation unit that uses dissolved air, and chemical dosing units. The most common biological treatment method is the activated sludge process.
The biggest problem in chemical treatment is the selection of the chemicals, which must be added to the wastewater in order to separate the dispersed pollutants. The problem nearly always culminates in finding a suitable coagulant, as this must be easy to handle, easy to store and prepare, and must achieve the required degree of removal of the pollutants.
We have observed that ferric sulphate with the right polymer is a very efficient coagulant for the above application.
Ferric sulphate is a primary coagulant in granular form based on trivalent iron (Fe+3), and can be dissolved easily to give a Fe content of 10.011.8percent. This coagulant works well in a wide range of pH and does not have the same operational problems as ferric chloride.
System efficiency
In a physical/chemical process for a CPP the ferric sulphate acts principally on colouring, TSS, BOD5 and turbidity, achieving the following removal efficiencies, considering that the effluent has the characteristics mentioned in Table 1.
It must be mentioned that the results shown in the above table are from processes where the plant has a dissolved air flotation unit (DAF) to clarify the wastewater, and where dosing of ferric sulphate (~20percent Fe3+) is between 300 and 350mg/l.
The sludge generated in the WWTPs where ferric sulphate is used as a coagulant is much easier to dewater than sludge generated with other products such as ferric chloride or aluminium sulphate.
We have noted that sludge coming from a physical/chemical treatment process where ferric sulphate is used requires only one cationic polymer to improve the dewatering characteristics, whereas in processes where other coagulants are used it normally requires three or more chemicals for dewatering the sludge.
Additional benefits
Some of the additional benefits for the use of ferric sulphate are:
* Precipitation of sulphur compounds.
* Increased efficiency in SS and BOD5 elimination.
* Reduction in energy consumption in the biological process.
* Lower corrosion and longer lifetime for the mechanical equipment in the plant compared to the use of ferric chloride.
* Less maintenance work and expense at the plant.
Water without chemicals. Fig. 1. shows the appearance of the water after passing through screening.The pollutants, normally suspended solids, grease and some blood, are all dispersed. Water with chemicals (Fig. 2). Coagulation and flocculation take place once the chemicals are added. This happens in the coagulation-flocculation pipe. Once flocculation has occurred we are ready to move to the next step, which is clarification in a dissolved air flotation unit.
Clarification. The DAF principle can be observed in Fig. 3. The sludge floats to the surface due to the dissolved air, and is removed from the surface by means of a continuous scraping device built into the top of the DAF. Clarified water is discharged or recycled at the plant for purposes such as washing crates or trucks. p
Ricardo Camacho and Luis Huerta are with OPTAR, SA de CV. For more information, visit www.kemira.com
