Instrumentation

Probably nothing has affected sewage treatment plant operation more than the introduction of adequate recording and control equipment. This, combined with research in the field of automatic laboratory testing, is bringing the automatic plant closer and closer to reality. One of these developments is the magnetic flowmeter. (See fig. No. 12.) By applying a uniform magnetic field around a section of stainless steel pipe, the volume of flow is measured by the electrical impulse created by the liquid flow through the magnetic field, thus the problem of clogging in conventional meters is eliminated. Another recent development is a meter to determine sludge density (percent solids) by the use of nuclear radiation. The meter can also be used (see fig. 13) to control sludge pumpage from primary sedimentation tanks to digesters, vacuum filters, or other sludge treatment processes. As shown in diagram, the pump would start at a predetermined interval and pump until the sludge reached a preselected minimum density. This scheme eliminates filling digesters with excess water or treating extra water with the sludge.

NOTE. Since design and construction of waste treatment works must necessarily be addressed to the problems of waste treatment and of plant operation and maintenance, much of the above applies also to the following subject which deals with the operation and maintenance of waste treatment works.

At Newport, R. 1., the influent flow includes a portion from a nearby Navy base. In order to determine how much of the total cost of treatment should be billed to the Navy, a Foxboro magnetic flow transmitter was installed (left) with a Foxboro Dynalog Recorder as shown (right).

FIGURE NO. 12

[From the Sewerage Manual, 1960 edition, public works publication

OPERATION AND MAINTENANCE OF SEWAGE AND WASTE

TREATMENT WORKS

GENERAL

Upon completion of the design and construction phases of waste water treatment installations, the owner must provide adequate funds for operation and maintenance. Although capital costs are generally financed by long-term bonds, the operation and maintenance costs must come from current budget funds which, in many cases, are subject to severe competition from other functions within the corporate organizational structure.

However, the problem of funds allocation must be equitably resolved to guarantee optimum operation and maintenance of the treatment facility, as well as to protect what generally represents a sizable investment to the community or industry. A listing of the major annual cost items would include competent personnel, operating controls, preventive maintenance, equipment replacement and modification, power, and other utilities.

THE PROBLEMS INVOLVED

The problems associated with the operation and maintenance of a waste treatment works are as varied and complex as other manufacturing industries having as their purpose, the converting of raw materials into valuable useable products.

Need for plant operators.-Procuring and retaining dedicated, capable personnel is a major problem in waste treatment operations even when funds are adequate. The lack of sufficient funds poses an intolerable situation which can result only in poor operation, lowered plant efficiency, and often deterioration of the physical plant.

A modern sewage treatment plant is a very complex system of structures, pipes, valves, meters, pumps, motors, and related appurtenances, all subject to wear and deterioration through use, and from the corrosive and abrasive nature of the sewage it processes. The tendency of humans to discard any and all materials to the sewerage system causes treatment plant problems, many of which can be handled more economically and practicable by methods other than the sewer.

Effects of industrial wastes.-Many types of industrial wastes have a deleterious effect on sewage treatment, and on the treatment plant and its appurtenances. Explosive and flammable substances should never be discharged to the sewerage system. Examples are drycleaning fluids, chemical solvents, and gasoline. The presence of oil interferes with sedimentation and vegetable oils exert a high oxygen demand on biological treatment processes.

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TRADITIONAL PRACTICES

Maintenance of plants.-The term maintenance may be defined as the art of keeping plant grounds, equipment, structures, and related facilities in suitable condition to perform the services for which they were designed. The effectiveness and efficiency of maintenance will reflect the concern of management and in turn the attitude of employees. A plant that is well maintained and operated will perform at a high efficiency, both from the standpoint of economy and waste treatment.

Maintenance should be carried out in a manner which prevents emergencies or unscheduled shutdowns by orderly preventive mantenance practices. Experience has shown that a basic maintenance program should include the following elements:

(1) A good housekeeping program that produces a clean, neat, and orderly plant in which the employees can efficiently carry out their responsibilities and which presents a pleasing appearance to the public supporting the facilities.

(2) A systematic plan for execution of daily operations, maintenance, inspections, and personnel activities.

(3) A system of records and recordkeeping which will reflect all necessary details of design, construction, operation, maintenance, costs, and future needs.

(4) A schedule of regular meetings of personnel for planning, training and instruction.

(5) A recognition of the accomplishments of personnel through an incentive awards and public information program.

NEW MATERIALS AND EQUIPMENT AND NEW USES OF OLDER MATERIALS AND EQUIPMENT

Maintenance problems and cost are reflected in the type or quality of materials available for construction of the structures and equipment, and the quality of materials from which the replacement parts are manufactured.

The materials most resistant to corrosion, erosion, and wear should be used within the economic limits of manufacture and general use. Normal economic cost limits must be disregarded when problems of difficult location, continuous plant operation, or plant and personnel safety are involved. The best use consideration in the selection of materials may often be the most economical in the long term due to savings in labor and out-of-service time.

Many new materials are coming into use which reduce maintenance requirements and improve plant operation and maintenance, and older materials are being adapted to new uses for these purposes. In addition to those discussed previously under the subject of design and construction of sewage and waste treatment works, they include:

Stainless steels.-Stainless steels in reactors, valves, tanks, shafts, pump impellers, pipes and fittings, and laboratory equipment where wear, rust, and corrosive materials are a problem. Tungsten carbide is being used for valve facings and buna N rubber for sludge pump stators because of the resistance of these materials to wear and corrosion.