Water Reuse

Conventional Wastewater Treatment Process

Although there are many variations of wastewater treatment plants, most will have the following steps: preliminary treatment, primary treatment, secondary treatment, tertiary treatment, disinfection, and solids handling.  Each of these steps will be reviewed below.

Preliminary Treatment

During Preliminary Treatment, the incoming raw sewage, or influent, is strained to removed all large objects that make their way into the sewer system.  These objects can be anything from rags and sticks to toys, cans and even snakes.  Generally bar screens, which come in a variety of shapes and sizes, are used to remove the items.  The influent flows across these screens, objects catch on the screens, are raised out of the water and are then raked (either mechanically or manually) off the screens. 

Another component of Preliminary Treatment is the grit channel where the velocity of the incoming wastewater is carefully controlled to allow sand, grit, and stones to settle to the bottom of the channel while keeping the majority of the suspended organic material in the water column.  The grit is removed from the channel, added to the larger objects removed by the bar screens, and taken to the landfill for disposal.

Preliminary Treatment is vital for preventing damage to pumps and other equipment in the remaining treatment stages.

Primary Treatment

Many plants have a sedimentation stage where the sewage is allowed to pass slowly through large tanks, commonly called primary clarifiers or primary sedimentation tanks.  The tanks are large enough that sludge can settle and floating material such as grease and oils can rise to the surface and be skimmed off.  The main purpose of primary treatment is to produce both a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed.  Primary clarifiers are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped to further sludge treatment stages.  The clarified water flows on to the next step of treatment.

Secondary Treatment

Secondary treatment processes can remove up to 90% of the organic matter in wastewater by using biological treatment processes.  The two most common conventional methods used to achieve secondary treatment are attached growth processes and suspended growth processes.

Attached Growth Processes  In attached growth (or fixed film) precesses, bacteria, algae, fungi and other microorganisms grow and multiply on the surface of stone or plastic media, forming a microbial growth or slime layer (biomass) on the media.  Wastewater passes over the media along with air to provide oxygen, and the bacteria consume most of the organic matter in the wastewater as food.  Attached growth process units include trickling filters, biotowers, and rotating biological contactors.

Suspended Growth Processes  In suspended growth processes, the microbial growth is suspended in an aerated water mixture where the air is pumped in, or the water is agitated sufficiently to allow oxygen transfer.  The suspended growth process speeds up the work of aerobic bacteria and other microorganisms that break down the organic matter in the sewage by providing a rich aerobic environmnet where the microorganisms supsended in the wastewater can work more efficiently.  In the aeration tank, wastewater is vigorously mixed with air and microorganisms acclimated to the wastewater in a suspension for several hours.  This allows the bacteria and other microorganisms to break down the organic matter in the wastewater.  Suspended growth process units include variations of activated sludge, oxidation ditches and sequencing batch reactors.

After biological treatment, the water is pumped to secondary clarifiers where any leftover solids and the microorganisms sink to the bottom.  These solids are handled separately from the supernatant which continues on to disinfection.

Tertiary Treatment

Tertiary treatment involves advanced treatment processes that generate a higher quality effluent than secondary treatment can produce.  These processes are vital for wastewater reuse; therefore they are discussed separately from conventional treatment in the Tertiary Treatment section.

Disinfection

The purpose of disinfection in the treatment of wastewater is to substantially reduce the number of microorganisms in the water to be discharged back into the environment and is almost always the final step in the treatment process regardless of the level or type of treatment used.  The effectiveness of disinfection depends on the quality of the water beign treated (e.g., cloudiness, pH, ammonia content, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables.  Cloudy water will be treated less successfully since solid matter can shield organisms.  Generally, short contact times, low doses, and high flows all prevent effective disinfection.  Common methods of disinfection include ozonation, chlorine, and ultraviolet light.

Chlorination remains the most common form of wastewater disinfection due to its low cost and long-term history of effectiveness.  One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment.  Residual chlorine or chloramines (formed by the combination of chlorine and ammonia) may also be capable of chlorinating organic material in the natural aquatic environment.  Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated adding to the complexity and cost of treatment.

Ultraviolet (UV) light can be used instead of chlorine.  Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it.  UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens making them incapable of reproduction.  The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement, and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation.

Ozonation is also becoming a popular alternative to chlorine.  Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a third oxygen atom becoming attached and forming O3.  Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with thereby destroying many pathogenic microorganisms.  Ozone is considered to be safer than chlorine because it is generated onsite as needed and does not have to be stored.  Ozonation also produces fewer disinfection by-products.  A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators.

Ozone is also useful at reducing the concentrations of iron, manganese, and sulfur by oxidizing these metals in water to form insoluble metal oxides or elemental sulfur.  The insoluble particles are then removed by filtration.  Ozonation is also effective at reducing or eliminating most taste and odor problems.

Solids Handling

Primary treatment and secondary biological processes concentrate waste organics into a sludge.  Methods for processing raw sludge include anaerobic digestion and mechanical dewatering by either belt-filter pressing or centrifugation.  Conventional methods of disposal are apllication as a fertilizer or soil conditioner on agricultural land, landfilling in a dedicated disposal site, or codisposal with municipal solid waste.  Since sludge disposal has little to do with water reuse, this subject will not be explored further in this website.

Sources:
http://en.wikipedia.org/wiki/Wastewater_treatment
http://www.epa.gov/OWM/primer.pdf
http://www.waterencyclopedia.com/Tw-Z/Wastewater-Treatment-and-Management.html
http://www.unitedwater.com/wastewtr.htm