Wastewater Treatment and Disposal

STOP - You must read Chapter 6 before doing this lesson

Introduction

The improper disposal of human excreta and sewage is one of the major factors threatening the health and comfort of people around the globe. This is so because very large number of different disease-producing organisms can be found in the fecal discharges of ill and apparently healthy persons. Proper disposal of sewage and other wastewater is necessary not only to protect the public health and prevent contamination of groundwater and surface water resources, but also to preserve fish and wildlife populations and avoid the creation of conditions that could detract from the attractiveness of a community, tourist establishment, resort, and recreational area.

The following basic criteria should be satisfied in design and operation of wastewater disposal system:

Prevention of microbiological, chemical, and physical pollution of water supplies.
Prevention of pollution of bathing and recreational areas.
Prevention of nuisance and unpleasant odors.
Prevention of human wastes and toxic chemicals for coming into contact with man, and wildlife.
Prevention of fly and mosquito breeding, and exclusion of rodents and other animals.
Strict adherence to standards for groundwater and surface water protection.
Compliance with federal, state and local regulations.

Definitions

Aerobic Bacteria - bacteria that require free dissolved oxygen for their growth; carbon, nitrogen, and phosphorus are required nutrients for growth.
Anaerobic Bacteria - they growth only in the absence of free dissolved oxygen. They obtain oxygen from breaking down complex organic substances. Some of the final products are ammonia, methane, carbon dioxide, and other.
Facultative Bacteria - thay have the ability to live under both aerobic and anaerobic conditions.

Municipal Sewage Treatment

The aim of sewage treatment is to improve the quality of wastewater to a level at which it can be discharge into a waterway without seriously disrupting the aquatic environment or causing human health problems in the form of waterborne disease. Achieving these goals requires killing pathogenic organism present in human wastes and to the greatest possible extent, removing organic wastes or converting them to inorganic forms so that after discharge they will not deplete the oxygen content of the receiving waters as they decompose.

To accomplish these ends, several levels of sewage treatment are necessary. These are:

Primary Treatment
Secondary Treatment
Tertiary Treatment

Generalized plan of a sewage treatment plant using either (or both) an ACTIVATED SLUDGE or a TRICKLING FILTRATION SYSTEM.

Primary Treatment - Mechanical Process

The waste water that goes into a treatment plant not only consists of human feces and urine, but also of wastes from laundry, bathing, garbage grinding, and dishwashing as well as all the miscellaneous articles which find their way into the sewer system such as sand, gravel, rubber balls, leaves, sticks, dead rats, etc.

Primary sewage treatment consist of several mechanical processes designed to remove the larger suspended solids through screening and sedimentation.

Screens - in general the incoming water first passes through one or more screens to remove large or floating objects.
Grinders - may be used to reduce the solids to a uniform size so they cannot damage or clog equipment.
Grit Chamber - it reduce the water flow to permit sand, gravel and other inorganic material to settle out.

Settling Tank in a Sewage Treatment Plant.
This is an empty settling tank. Note the sweeping arm at the bottom of the tank that slowly rotates and sweeps the settled sludge into the center where it is removed.

Primary Sedimentation Tank - the water enters a sedimentation tank where it is hold for several hours to allow suspended particles to settle out or precipitate. The solid material which settled out (SLUDGE) in the sedimentation tank is regularly removed, dried, and disposed off by one of several methods. (See Sludge treatment)

Following several hours in the primary sedimentation tank the water enters secondary treatment.

Primary treatment is better than no treatment at all, but it is not good enough to prevent degradation of the receiving waters. Also, primary treatment alone is illegal. The EPA does not consider primary treatment by itself an adequate level of sewage treatment. Primary treatment removed approximately 50 to 65 % of suspended solids, and BOD is reduced by about 25 to 40 %.

Secondary Treatment - Biological Process

Primary treatment is based upon physical and mechanical methods of removing suspended solids from waste water, secondary treatment depends of biological processes, similar to naturally-occurring decomposition but greatly accelerated, to digest organic wastes. Microorganism, predominantly aerobic bacteria, are utilized in the presence of an abundant oxygen supply to breakdown organic materials into inorganic carbon dioxide, water, and minerals.

These can be accomplished by means of:

Trickling Filtration - beds of crushed stones whose surface are covered with microbial slime consisting of bacteria, protozoa, nematodes, molds, algae, as well as insect larvae, which absorb the organic material as the waste water is spray over the surface of the rocks. The oxidized products of microbial metabolism flows out with the effluent into the secondary settling tank along with chunks of slime that breaks off from time to time.

Trickling filter tank design.
The effluent from the primary settling tank flows into the spraying arms, which rotate slowly over the rocks or other inert material. As the effluent passes through the air as a fine spray it is saturated with oxygen. The organic matter is removed from the sewage effluent as it trickles over the rocks.

Activated Sludge Process - this involves "seeding" a tank of sewage water with bacteria from the sludge, pumping compressed air into the mixture and agitating it for 4 to 10 hours. During this time aerobic bacteria adsorb most of the suspended solids from the surface of the sludge particles and oxidize the organic material.

Design of an aeration tank for activated sludge.
The air, under pressure is pumped into the tanks and vigorously mixed with the sewage to insure the material always remain aerobic. On the far left, some of the activated sludge is mixed with the fresh, incoming, sewage from the primary settling tank. The activated sludge contains hungry microbes that immediately chow-down on all that yummy fresh sewage.

Picture of aeration tanks of activated sludge at a Sewage Treatment Plant.
This sewage treatment plant (STP) operates 6 aeration tanks. Different tanks can be shut down at different times. This flexibility is another advantage to the activated sludge system. The building in the background houses the large air pumps and the other pumps for moving the sewage around through the system.

After the process is completed, the sludge is separated from the remaining liquid by settling on the secondary sedimentation tank. Most of the sludge, consisting primarily of masses of bacteria, is removed, but some must be retained and fed back into the incoming sewage to perpetuate the process. (See figure below).

After wastewater has passed through both primary and secondary treatment, the level of suspended solids and of BOD has been reduced by about 90 percent. Cold weather can reduced the efficiency of pollutant reduction because it slows the metabolic rate of the microorganisms on which secondary treatments depends.

Secondary treatment is not effective in removing viruses, heavy metals, dissolved minerals, and certain other chemicals. Federal law requires that all sewage treatment plants provide at least secondary wastewater treatment.

Tertiary Treatment - Advanced Wastewater Treatment

In situations where effluent from the secondary treatment still contains substances which are causing water quality problems; when the sheer volume of effluent is large enough that the remaining 10% of suspended solids and BOD are sufficient to initiate eutrophication, or if the treated wastewater is to be used for purposes of drinking, irrigation or recreation then a third level of sewage treatment is required.

Advanced wastewater treatment involves either one or a combination of several biological, chemical or physical processes designed to remove such pollutants as phosphates, nitrates, ammonia, and organic chemicals. It also, further reduces the concentration of remaining suspended solids and BOD to about 1% of that present in raw sewage.

Examples of methods of tertiary treatments are:

Chemical coagulation followed by filtration or sedimentation
Activated carbon adsorption
Ion exchange
Oxidation ponds and aerated lagoons
Reverse osmosis

Chlorination

Chlorine treatment of effluent.
The photograph in the left shows the large chlorine tanks used on STP. The chlorine is pumped as a liquid and metered into the effluent from the secondary sedimentation tank. It then flows through the chlorine basins, shown in the right, while the chlorine acts to kill most of the remaining pathogens. The final effluent is releases to the environment, a river, creek, lake or ocean.

Since a substantial number of live organism still remain in the wastewater after primary and secondary treatment are completed, it has been standard procedure for many years to add chlorine to the treated effluent before it is discharge into a river, creek, lake or ocean. Chorination policies has change because:

chlorine is effective in killing bacteria but less so in relation to viruses,
chlorine is very destructive of fish and other form of aquatic life,
chlorine treatment is expensive and poses safety problems at the treatment plant,
chlorine disinfects only a fraction of the waste in streams because bacteria can come from runoff.

Sludge Disposal

Sewage Treatment Plant dried sludge bed.
This dried sludge bed is about 2 acres in size and approximately 6 feet deep. Some STP produce sludge of excellent quality that is safe to use as a fertilizer on farms. Other may content high concentrations of heavy metals.

Anything that settles out and is subsequently collected during the sewage treatment process is called sludge. All the sludge is collected in one place and treated separately from the rest of the sewage material. Sludge it self can be a pollutant if improperly managed. Sludge treatment involves an anaerobic process that converts approximately 50% of the organic sludge mainly into methane gas, and some carbon dioxide. To do this the sludge is pumped into large enclosed tanks. In the tanks it is allow to ferment for several weeks while being gently stirred.

General design and appearance of an anaerobic sludge digestion tank.
During this time anaerobic bacteria convert 50% of the organic matter into methane gas. This

Anaerobic sludge digestion tank at a STP.
The sludge from various parts of the STP are pumped into the thicking tank where it further settles out. The thick concentrate is them pumped into the digestion tank and fermented for several weeks by anaerobic digestion.
methane gas can be use for heating purpose, or as fuel for vehicles. After this the sludge has to be dewatered by one of several methods such as drying on sand beds, vacuum filtration, centrifugation, etc. After that it may be put in a landfill, incinerated or composted if it is not contaminated with heavy metals.

Disposing of large quantities of sludge present a really interesting challenge. It can be burned (contributing to air pollution), buried into landfills (contributing to ground water pollution) or dumped into the ocean (contributing to their degradation). If it is safe (no heavy metals, toxic organic chemicals and the pathogens killed) it makes an excellent fertilizer for lawns and farms. Sludge treatment and disposal account for over 50% of the operating costs of a typical sewage treatment plant.

Water Environmental Federation web site.

EPA: How water treatment works.