Sewage Treatment/Cultural Eutrophication

SEPTIC TANKS

Septic tanks are used in areas where there are no combined sewers.

Septic tanks (cesspools) must be emptied each year. Old tanks can crack and leak coliform bacteria into surrounding soil, groundwater, and surface waters.

Figure 9-28Page 196

Householdwastewater

Perforated pipe

Distribution box (optional)

Septic tank with manhole (for cleanout)

Drainfield Vent pipe

Non-perforated pipe

Gravel orcrushedstone

SEWAGE TREATMENT• Sewage treatment

means removing impurities so that the remaining waste water can be safely returned to the surface waters (river, bay, ocean) and become part of the natural water cycle again.

• sewage treatment separates solids from liquids by physical processes and purifies the liquid by biological and chemical processes

PRIMARY TREATMENT• solids like wood,

paper, rags and plastic are removed by screens, washed, dried and taken away for safe disposal at a licensed waste tip. Grit and sand, which would damage pumps, are also removed by settling tanks and disposed of in a similar way.

PRIMARY TREATMENT

• the remaining solids are separated from the liquid by passing the sewage through large settlement tanks, where most of the solid material sinks to the bottom. About 70% of solids settle out at this stage and are referred to as sludge. The sludge is used on farms after further treatment called sludge treatment.

SLUDGE TREATMENT• Sludge is an excellent soil conditioner and is used as a fertilizer on

farmland. However, it needs additional treatment to make it suitable. This treatment is called anaerobic digestion and takes place in large, enclosed tanks. NYC sludge has too many heavy metals (Hg, Pb, Cd, Cu) and may not be used as a soil amendment for food resources in NY State.

• The rate of digestion is increased by heating the sludge to a temperature where naturally occurring bacteria (microorganisms) respond to these comfortable conditions and feed on other bacteria. On cooling, the well-fed bacteria die off, and the sludge is suitable for use on agricultural land.

• A by-product of the sludge digestion process is methane gas. This can be burned to produce electricity. The electricity can be used to heat more sludge or to provide heat and light for the treatment works. Sometimes more energy is produced than is required. The surplus is sold to local electric companies.

SECONDARY TREATMENT• a biological process which relies

on naturally occurring microorganisms acting to break down organic material and purify the liquid.

• In a simple sewage treatment process, micro-organisms are encouraged to grow on stones over which the sewage is trickled. They feed on the bacteria in the sewage and purify the water. These treatment units are called percolating filters.

SECONDARY TREATMENT

• The rate of this process can be increased by pumping air into tanks of sewage where the aerobic digesters float freely and feed on the bacteria. These treatment units are called aeration tanks.

• Following either form of secondary treatment, the waste water is settled in tanks to separate the biological sludge from the

purified waste water.

Raw sewagefrom sewers

Bar screenGritchamber Settling tank Aeration tank Settling tank

Chlorinedisinfection tank

Sludge

Sludge digester

Activated sludge

Air pump

(kills bacteria)

To river, lake,or ocean

Sludge drying bed

Disposed of in landfill orocean or applied to cropland,pasture, or rangeland

Primary Secondary

(Sludge cake or pellets)

TERTIARY TREATMENT• Extra treatment is

needed to give the waste water a final "polish". This is known as tertiary treatment. Various methods may be used, including sand filters, reed beds or grass plots (artificial treatment wetlands). Disinfection, using ultra violet light to kill bacteria, is another method, and is being used at a number of coastal sewage

treatment plants.

Rotating Drum Treatment

Sand Filtration

Carbon Filtration

There are two principal mechanisms by which activated carbon removes contaminants from water; absorption, and catalytic reduction, a process involving the attraction of negatively charged contaminant ions to the positively-charged activated carbon. Organic compounds are removed by absorption and residual disinfectants such as chlorine and chloramines are removed by catalytic reduction.

Types of

organisms

Dissolved

oxygen

(ppm)

Biological

oxygen

demand

Normal clean water organisms

(Trout, perch, bass,

mayfly, stonefly)

8 ppm (mg/L)

Trash fish

(carp, gar,

leeches)

Fish absent,

fungi, sludge

worms,

bacteria

(anaerobic)

Trash fish

(carp, gar,

leeches)

Normal clean water organisms

(Trout, perch, bass,

mayfly, stonefly)

8 ppm

Clean ZoneDecomposition

Zone

Septic

Zone

Recovery

Zone

Clean

Zone

IMPACTS FROM ORGANIC WASTE

POLLUTION FROM SEWAGE

SEWAGE:

•poses a threat to public health because it carries disease-causing agents (cholera bacteria, hepatitis, Eschericia coli (coliform bacteria).

•Causes nutrient-loading (organically-rich) into surface waters.

•Causes an increase in BOD (biological oxygen demand) or BOD5 over five day period, which causes a decrease in

MONITORING SEWAGE

Standard test for for total coliform (TCOL) and fecal coliform (FCOL) bacteria. The number of colonies formed are counted.

STANDARDS: (EPA)

1. Drinking Water – 1 coliform bacteria:100 mL of water

2. Swimming Water – 200 coliform bacteria: 100 mL of water

DISPOSAL OF SEWAGE SLUDGE

1. ANAEROBIC DIGESTION – anaerobic bacteria break down organics into methane gas (NH4) and CO2. Methane is trapped and used to heat the digester to 95F.

END PRODUCT = Soil conditioner for gardens (humus).

2. FERTILIZER – sludge is rich in plant nutrients and can be dried (pelletized) and sold as a fertilizer.

PROBLEM: Combined sewer (industrial, residential and storm water combined may be high in heavy metals and PCB’s (NYC SLUDGE!)

WATER POLLUTIONCONTROL LAWS

1988 – Ocean Dumping Ban Act – barred ocean dumping of sewage sludge at the 200 mile marker. All cities were in compliance by 1988 EXCEPT for NYC, who were permitted to dump until June, 1992.

1972 – Water Pollution Control Act – EPA established regulations for the discharge of pollutants in the USA. It gave the EPA authority to implement pollution control programs and set ambient water quality standards for all contaminants entering surface waters. It also funded the construction of sewage treatment plants.

WATER POLLUTION CONTROL LAWS

• The Federal Water Pollution Control Act was amended in 1977 and came to be known as the Clean Water Act. The ACT does NOT address water quantity or groundwater, ONLY water quality. Initially addressed point source, since the 1980’s has come to address nonpoint source issues as well.

WATER POLLUTION CONTROL LAWS

• Great Lakes Critical Programs Act, 1990, put in place part of the Great Lakes Water Quality Agreement between the USA and Canada. This law required the EPA to establish water quality criteria to address 29 toxic pollutants with maximum levels that are safe for humans, wildlife, and aquatic life.

WAYS TO REDUCE CULTURAL EUTROPHICATION

• Advanced waste treatment (sewage and industrial)• Regulate detergents cleaning products for phosphates.

They have been regulated since the 1970’s on Long Island!

• Promote soil conservation and preservation of wetlands to capture and filter pollutants before they enter surface waters.

• Regulate fertilizer, pesticide, agricultural and livestock runoff to control Nitrogen, phosphate, and PAH’s.

• Create policies and economic incentives for doing these things!