Waste Water Management
Industrial operations produce a liquid product that almost always must be treated before being returned to the environment. There are three different groups of wastewater to be considered.
Classifications of Waste Water
These waters are produced by the mere acts of living such as using the bathroom, doing laundry, or washing the dishes. These wastes are normally handled by the sanitation department, which eliminates pathogens before disposal.
These waters are produced by some industrial processes and include the undesired liquid product of any unit operation. The major concern with these wastes is the reactions that may occur with the environment being either direct or indirect. Some may rob oxygen from the environment, while others may be toxic.
These waters are produced as a result of some sort of heat exchanger where heat is removed from the product. Waters can be used once or recycled. Recycling creates the necessity for periodic cleaning, where at least some may be released into the environment. This is the least dangerous the waste waters, but because of leaks within the process, could contaminate the surroundings. This type of waste must also be monitored and often treated, and is also a major factor in thermal pollution of water sources.
The Characteristics of Waste Water
These are directly regulated on a basis of toxicity. Government regulation required permits and inspections. All priority pollutants are toxic. They include heavy metals, PCB's and various forms of benzene compounds. For a complete listing see Industrial Water Pollution Control, by Eckenfelder, W. W. Jr., 2nd Edition, McGraw Hill, NY, 1989.
These can vary from proteins to solvents, and most industries produce several different streams. It is important to note in design that side reactions also need to be considered. One way to measure the "strength" of an organic was is the 5-day biochemical oxygen demand (BOD5). This measures the demand for dissolved oxygen by the microbes that attack the organics within a specified volume over a 5-day period.
These are a combination of the inorganics coming in and the direct result of any reaction that produces inorganics, or the addition of inorganics within a process. It is important to note that a balance is needed before this liquid is returned to the environment.
Chemical reactions and additions to the process tend to either increase or decrease the pH and alkalinity of a solution. The pH must be regulated to between 6 and 9 for little impact upon discharge.
Due to biological treatment, if required, the temperature of wastewater should be kept between 10 and 30 C if possible. This is due to the optimum temperature that microorganisms function at.
Aquatic life requires a certain amount of D.O. to survive. Therefore there are maximum levels, which can also be lethal, and minimum levels of D.O. that must be met before discharge can occur.
This includes volatile and nonvolatile solids either suspended or dissolved in the wastewater. These may be either organics or inorganics. A certain percentage of total solids must be met before discharge.
These may cause biological activity in the discharge area to increase, which will inherently lower the amount of D.O. in the system. This directly affects the amount of life that can be sustained within the area.
These compounds may interfere with oxygen transfer within aquatic life. It also leads to a poor appearance due to the insolubility.
There isn't a standard design to treat all wastewaters due to the characteristics of the process specific waste. Each site requires a design specific to the process at hand, but some combination of the following pretreatment, primary treatment, secondary treatment, and processing are normally used. As a rule of thumb, we would like to separate several pollutants in one step and use several steps to increase the degree of treatment.
The Pretreatment Process
This is very widely used as it may drastically cut down overall treatment costs.
This is the process of adjusting the concentration of wastes, flow of wastes, mixing, oxygen characteristics, temperature, and so on.
This is the process of adding materials, mixing and adjusting pH measurements for the bulk of the discharge to adequate values.
Due to solubility characteristics, we can separate oils by gravity, or something as simple as skimming. We can also add chemicals to break down or separate oils.
These must be separated from the discharge to insure that federal guidelines are met. Chemical addition and reaction can often aid in the separation.
The Primary Treatment Process
The goal of primary treatment is to remove solids by chemical or physical methods.
Various types are currently being used to filter particulate matter from the solution. Machines may be used to keep the screens clean and clogging is a frequent problem.
Here hard inert particles are separated using a mixing pattern where air provides a circular current and inerts settle to the bottom of the chamber, are picked up in a screw drive conveyer and are carried off for disposal. This process resembles a centrifuge, and provides fast separation.
This process is used for materials with slower settling times. They are designed on the basis of the retention time necessary for adequate waste removal. Operation is essentially the same as for the grit chamber, except in flow rates are much slower, and a scum layer can be removed from the surface due to poor mixing.
A coagulant is added to suspend solids for removal by sedimentation. However, it increases the amount of sludge that must be processed.
The Secondary Treatment Process
The main concept behind secondary treatment is the use of microorganisms to break down components of the waste into less damaging forms.
The basis of biotreatment is directly related to many of the concepts provided in the course work of Introduction to Biochemical Engineering. Here the kinetics and stoichiometry are analyzed, tests and experiments are performed and then the process is scaled up. The biomass used is completely dependent on the type of sludge being processed.
This is a treatment process by which air is provided to enable aerobic growth. Removal of soluble wastes can be accomplished by biological metabolism, adsorption, and collection by the cell population.
Blowers, spargers, and many other mechanical means may be employed to provide adequate oxygen to sustain biological activity.
These are used as with primary treatment to remove solids from the treated liquid. A recycle stream may or may not be employed depending on the biomass composition of removed stream.
This process usually requires more time than the aerobic process, but can successfully remove the amount of nitrates in the waste stream.
These are low cost, suspended growth, no recycle reactors that can be aerated, non-aerated, or scum layer covered, anaerobic processes. They can be linked in series, with recycle streams if desired.
Here biomass remains attached to the packing in a reactor, allowing a flow type system, which can decrease retention time, but decrease the degree of treatment especially with aerobic processes.
These are fixed-medium bioreactors where liquid is spread over the surface of a solid where microbes are growing. At the surface aerobic digestion is found, and deeper yet anaerobic digestion occurs. Rocks were originally used, but now plastics have been found that allow for a greater wastewater depth, while still allowing for adequate treatment.
This is another form of trickling filter where rotating disks are immersed in a tank or stream of waste. As the disks leaves the water, a thin film forms on the surface, where microbes on the disk oxidize the organics.
Packed Bed Fixed Film systems, Fluidized Beds
Physical Chemical Treatment
These are the processes after all methods of treatment have been completed.
If sludge has a low enough water composition we can reduce the volume by combustion. Air and a startup fuel are normally added, making cost large. Using the released heat to further drying of the incoming sludge, or using the heat to produce energy can offset some of the costs. The ash is normally buried in a landfill.
These are non-leaching storage areas which are buried. They must be monitored regularly to prevent environmental damage.
Because sludges are rich in nutrients, using them as a fertilizer is a viable method of disposal. Agricultural areas normally benefit, as does the industry.