Chemical treatment is included under primary treatment because it involves chemical and physical processes as distinct from the biological processes which are the basis of secondary treatment. Chemical addition has application: (1) in the treatment of industrial wastes which are sometimes difficult to decompose by biological means, (2) where discharge to receiving waters require a higher degree of treatment than primary units can achieve, but secondary treatment is not warranted; (3) after secondary treatment when a higher degree of treatment is demanded (often referred to as tertiary treatment).
The exact point of chemical application is dependent on design considerations as well as permit requirements. We are concerned, here, with chemical treatment and its application to primary treatment.
Chemical treatment involves the addition of one or more chemicals to wastewater to produce floc, which is an insoluble chemical compound that adsorbs colloidal matter, enmeshes non-settleable suspended solids and settles more readily. The precipitating chemical also dissociates or ionizes in the wastewater and neutralizes the electric charges held by colloidal particles causing them to coagulate to form larger readily settleable solids. The chemicals most widely used are aluminum sulfate (alum), ferric chloride, lime and polymers.
A chemical treatment plant usually has the following features :
- Preliminary devices - screens, grit chambers, etc.
- Chemical feeders.
- Flash mixing units.
- Flocculation tanks.
- Sedimentation tanks such as have already been described.
- Increased Facilities for the treatment and disposal of sludge.
A large variety of units to feed chemicals, either dry or in solution, in controlled amounts, are made by a number of manufacturers.
The chemicals, when added to sewage, must be thoroughly and quickly mixed with it to provide complete and uniform reactions. This is accomplished by violent agitation for a short period of time either by mechanical or hydraulic methods. This agitation is carried on in special tanks, in sections of other tanks, or in the piping system. Such mixing devices are made by a number of manufacturers.
After the chemical is mixed with sewage it is gently agitated for 15 to 30 minutes to foster the coagulation of particles.
If BOD reduction is desired, the agitation time may be increased to 45 minutes. The colloidal and suspended solids meet and adhere together in large flocculant masses which settle readily in the sedimentation tank. Different types of equipment to accomplish this purpose are made by a number of manufacturers and must be designed to operate at varying speeds as
determined by the actual process characteristics.
The volume of sludge obtained by chemical treatment is greater than with standard primary treatment, necessitating a comparable increase in the capacity of the sludge handling facilities and in the cost of sludge treatment and disposal.
Chemical treatment can effect a reduction up to 90 percent in suspended solids and up to 70 percent in the BOD. It is well adapted to intermittent operation and has value in sewage treatment to reduce pollution of streams during periods of low
flow or to lessen pollution of bathing beaches and recreational waters during months when these facilities are in use.
It is of value also for the treatment of sewage containing high concentrations of industrial wastes which will inhibit biological life and interfere with secondary treatment processes. Operational costs are high due to increase in operator's time, chemical usage and the quantities of sludge to be treated and disposed of.