DAF PRETREATMENT

 

 

CHARACTERIZATION OF EFFLUENT STREAM

 

The effluent stream from most food related processing plants contain high levels of BOD which consist of two (2) basic components as follows:

 

                                                                1.             Suspended Organic Matter

                                                                2.             Dissolved Organic Matter

 

The Suspended Organic Matter is that which makes the water opaque or unclear.  It consists of particles of the product produced in the plant process.  These particles can be large enough to be removed by screening or so small as to be individually invisible to the unaided eye.  The majority of the suspended solids in the water fall into the latter category.  It is the very small size which keeps them in suspension.

 

The Dissolved Organic Matter is that which is not visible in the water, such as proteins, sugars and other inclusions which readily dissolve in water.  These can not be seen in the water at all, as they are in an ionized state.

 

Conventional water treatment consists of some sort of screening followed by a biological reduction process.  This biological process can either be natural ( utilizing existing natural resources ) or man made biological systems such as a typical municipal wastewater treatment facility.  When industrial wastewater is treated in a biological system, potential problems exist due to the typical high levels of suspended solids ( TSS ), Oil & Grease ( O&G ) and biological oxygen demand ( BOD ).  Biological systems do well with low levels of TSS, O&G and moderate levels of BOD.  They are easily overwhelmed with high levels of organic TSS and O&G.  Meat, Fish and Poultry Processing Plants are usually required to provide pretreatment due to this sensitivity of biological systems to high levels of organic TSS and O&G.  While meat and poultry plants have routinely provided wastewater pre-treatment for many years, this technology has been slow in coming to the fish industry because of their typical location near large bays and the ocean.  This environment has served well as a natural biological treatment method.

 

EXPECTATION OF A DISSOLVED AIR FLOTATION SYSTEM

 

Dissolved Air Flotation ( DAF ) is the industry standard system for removal of suspended solids and oil & grease in industrial wastewater effluent streams.  A typical water stream discharged from a meat, fish or poultry processing plant consists of the following.

 

CONSTITUENT IN THE WATER                   ANALYTICAL                      REMOVAL METHOD

                                                                                DESCRIPTION

 

Blood                                                                     TSS / BOD                             DAF Physical / Chemical

Free Animal Fat                                                    O&G/BOD                             DAF or gravity clarifier

Emulsified Animal Fat                                         O&G/BOD                             DAF Physical / Chemical

Particles of Tissue                                               TSS/BOD                               DAF Physical / Chemical

Salts and other soluble inorganics                    TSS ( No BOD )                    Evaporation / Reverse  Osmosis

Soluble Organics                                                  TDS/BOD                              Biological / Reverse Osmosis

( Salts and Soluble Organics Combined )         TDS

Residual Chemicals and Dirt                              TDS/BOD                              DAF Physical / Chemical

 

 

The above chart helps to show the treatment methods used to remove the more significant pollutants in wastewater from a meat, fish or poultry plant.  In primary processing plants ( slaughter ), the blood content tends to be quite high compared to further processing plants.

It can be readily seen from the chart, that most of the pollution in the water is suspended in nature and not dissolved.  In meat, fish and poultry, a minor portion of the protein from the animal can be dissolved, but most is in the form of small particles ( after screening ) such as individual blood cells, particles of flesh and animal fats.  The proportion of suspended vs. dissolved organic material in the stream depends on several factors.

 

                                                                1.             Type of product

                                                                2.             Type of process in the plant

                                                                3.             Operation of the plant

                                                                4.             Point source removal

 

In a typical meat, fish or poultry plant, the dissolved air flotation system is designed to remove nearly all of the suspended solids and fats from the wastewater stream prior to discharge to a biological system.  In the process, coagulation and flocculation chemicals are used to break emulsions and suspensions so that the oils and suspended solids can be separated from the water.  There are many different chemical processes used, and the one selected depends on a combination of factors, evaluated on a case by case basis.

 

 

COMPONENT                                                     EXPECTED REDUCTION

 

TSS ( Suspended Solids )                                   Greater than 90%

O&G ( Oils and Grease )                                   Greater than 90%

BOD ( Biological Oxygen Demand )                65% to 85% ( depending on proportion of soluble BOD to Insoluble BOD )

Salts ( Specific Conductance )                          0%

 

 

EFFECT OF DAF TREATMENT ON BIOLOGICAL TREATMENT SYSTEMS

 

SPRAY FIELDS

 

When the water from a meat, poultry or fish process plant is sent directly to a spray field without pretreatment, the high level of suspended solids and fats can interfere with the biological treatment expected from the bacteria in the soil.  The fats tend to seal the surface of the soil and prevent percolation.  This results in ponding on the surface and prevents oxygen from reaching the soil.  At the same time, the solids settle at the surface, filtered by the soil and are slow to degrade.  All of this creates an anaerobic condition, both on the surface within the solids, and in the subsurface where oxygen is excluded.  The result is poor growing conditions, ponding and offensive odor production.

 

When DAF pretreatment is employed, the oil & grease as well as the suspended solids are removed prior to reaching the spray fields and the resulting clarified water contains primarily dissolved organics which will readily penetrate the soil.  In a well maintained field, oxygen can reach into the soil and the organic material in the water is easily consumed by the bacteria present in the soil, converting the material into a form readily used by the plant material growing in the field.

 

BIOLOGICAL TREATMENT SYSTEM

 

A typical biological system is designed to treat dissolved organic material.  The bacterial in the system are capable of producing enzymes to break down some suspended organic material and oils, up to levels of about 200 mg/l TSS and 100 mg/l O&G.  When levels of suspended solids and oils go much higher than this, the biological system is overwhelmed and works poorly.  A typical biological systems can tolerate increased levels of soluble BOD much better than increased levels of suspended solids and oils.  For this reason, a DAF is employed upstream of biological systems in the meat, poultry and fish industries, whether the system is a part of the production plant or is a municipal treatment plant.

 

CONCLUSION

 

Regardless of the downstream biological treatment method used, its performance will be greatly enhanced by pre-treatment of the plant effluent.  The most proven method for removing suspended solids and oils & grease in most food processing plants is physical / chemical separation utilizing dissolved air flotation.