Sewage Treatment Stages

Preliminary Treatment

It is necessary to remove inorganic solids from the waste stream prior to treatment to ensure that they don’t clog and damage pumping and other equipment employed in the treatment plant. Being inorganic there is also no benefit to having this material pass through the plant as it will remain unchanged after the treatment process. The material is removed by means of screens that are either manually (for small plants) or automatically (in large plants) raked to remove the trapped material for disposal to a landfill or incineration.

Sand is removed from the waste stream by means of a grit channel (or de -gritter) in which the velocity of the influent wastewater is managed to permit sand, grit and stones to settle, whilst ensuring that the organic material remains in suspension in the water. A sand washer, or grit classifier, may be employed to remove the sand to a waste bin, whereafter the sand is usually disposed of in a landfill.

Primary Treatment

After the grit and inorganic material has been removed from the effluent, the waste flow passes through large, typically circular sedimentation tanks called primary clarifiers to allow sludge to settle and floating material such as fats and oil to be skimmed off the surface and disposed of separately. The sludge is also drawn off and either remixed with the rest of the wastewater for secondary treatment or separated to be disposed of or treated on its own. The sludge is worked towards a hopper at the base of the in the primary clarifier tank by a continuously rotating mechanical scraper.

Secondary Treatment

During the secondary treatment stage the organic matter in the wastewater is biologically degraded using microorganisms such as bacteria and protozoa in conditions that have been designed and controlled specifically to optimize the organic consumption rates of these organisms. Different organisms required different environmental conditions with the most efficient organisms employed for organic degradation thriving under aerobic conditions while organisms requiring anaerobic (without free and chemically bound oxygen) conditions are often also encouraged to grow as part of the treatment process. Nutrient removal forms an important part of secondary treatment and an anoxic (without free oxygen) environment is created to encourage the growth of organisms that thrive under these particular conditions. Secondary treatment can be classified as either fixed-film or suspended-growth treatment. In a fixed-film treatment process the biomass of organisms grows on media that the wastewater passes over. Trickling filters and rotating biological contactors are examples of this type of process. Fixed-film systems tend to be simple to operate and low in maintenance and are usually more adept at coping with drastic changes in the loading of organic waste material. They can also provide higher removal rates for organic material and suspended solids than suspended growth systems. One drawback of fixed-film systems, however is that they tend to take up a lot of space and require a large plant size for a particular waste flow. In suspended-growth systems such as activated sludge, the biomass is continually mixed and the treatment plant can be a great deal smaller than a fixed-film plant treating a similar waste flow.

Fixed-film processes

Trickling Filters
One of the oldest methods of treating sewage, in a trickling filter settled wastewater is distributed through perforated rotating arms radiating from a central pivot over a deep bed of coke (carbonized coal), limestone chips or, more recently, strips of plastic media. A high surface area of media is required to allow a biofilm to develop under mostly aerobic conditions and the wastewater needs to pass over as much media as possibly in its route to a collection drain at the base. The biofilm typically consists of bacteria, protozoa and fungi that in turn is eaten by insect larvae and worms which help maintain an optimal thickness of the biofilm. Although a trickling filter can accommodate short periods of higher concentration organic wastewater, it is inherently restrictive in its ability to treat varying flows as the biofilm needs time to develop and can therefore be a problem when overloaded. Once overloaded the biofilm thickness increases to the point of clogging the filter and the effluent gets trapped in ponds or short circuits to the drain untreated.

Rotating biological contactors
Originally used in Germany in the early 1960’s, rotating biological contactors (RBCs) are mechanical devices consisting of rotating disks on which the organisms used for treating the wastewater attach themselves and are constantly being re-immersed in the waste flow to break down the organic material. An advantage of RBCs is that they are more capable of withstanding increases in flow rates than trickling filters in that more disks can be added or additional channels containing the disks can be brought into use in anticipation of an increased waste flow. The microorganisms get their oxygen from the atmosphere as the disks, which are partially submerged in the wastewater, rotate. Excessive build-up of biomass sloughs off the disks due to the shear forces generated through rotation and the wastewater exiting the plant then passes through settling tanks where the sludge settles out for further treatment or disposal.

Biological aerated filters
A Biological Aerated Filter (BAF) is a fixed-film treatment process that usually consists of a reactor filled with a filter media that is either in suspension or supported by a layer of stones at the bottom of the reactor. The action of a BAF is the combination of filtration with organic material reduction, nitrification and sometimes denitrification. The biomass is supported on the media which also serves to filter suspended solids and the filter is operated in either an up-flow or down-flow configuration. Disadvantages of this type of filter are its tendency to get clogged very easily, the high maintenance required to keep it operational and its inability to treat large variations in flow.

Suspended-Film Processes

Surface-aerated basins
One of the simplest forms of suspended-growth treatment processes, a surface aerated basin is typically 1.5 to 5m in depth depending on the type of aeration device used. Aeration is typically achieved either by a motor-driven device floating on the surface of the basin which oxygenates the water from above or by means of a network of pipes on the bottom of the basin through which air is pumped for distribution by means of air diffusers. The action of both types of aeration methods is to also keep the biomass permanently suspended in the wastewater and o constantly keep the basin well mixed. Surface aerated basins have the disadvantage that they take up a lot of ground space and are not nearly as efficient in treating wastewater as the activated sludge process.

Activated sludge
By far the most common form of sewage treatment employed world-wide today, activated sludge is a suspended-film treatment system whereby the wastewater passes through several processes to remove the organic material and nutrients. These processes typically include an aerobic, anoxic and sometimes anaerobic zone in the plant and the wastewater is recycled between the different zones to achieve the right exposure time for the necessary organisms treating the water, depending on the concentrations of food available for the organisms. Oxygen is added to the wastewater in the aerobic zone either by surface aeration or via mechanical blowers that introduce the oxygen to the water by means of specially designed diffused air devices. Nitrification and denitrification for removal of nitrogen from the wastewater is achieved by managing the required recycle rates between different parts of the plant and additional phosphorous uptake, where necessary, is achieved in an anaerobic zone. Sludge is drawn off from the plant on a regular basis to maintain the required retention time of the biomass to have sufficient contact with the wastewater, hence the term “activated sludge”. This sludge is then either treated further or disposed of to landfill or for soil conditioning after dewatering. An activated sludge plant can be a highly efficient means of treating sewage with the different processes controlled by various electro-mechanical pieces of equipment and monitoring devices. These plants have the advantage that they take up very little space compared to other treatment methods and yet they can also produce the highest quality effluent.

Membrane bioreactors
A membrane bioreactor (MBR) plant combines the activated sludge process with a low pressure microfiltration membrane to separate the sludge from the wastewater after treatment. This solves the problem of a poorly settling sludge which is sometimes encountered in activated sludge treatment. It is also not normally necessary to use clarification and tertiary treatment for the effluent once it has passed through the membrane. The capital costs of building an MBR plant are usually higher than an activated sludge treatment plant and operational costs are also typically higher, however the advantages include a plant that takes up less space than other treatment methods and an effluent that is of very high quality.

Package Plants
A package plant is an effluent treatment plant that is typically pre-assembled in kit form off site in a factory so that it can be brought to a location and set to work in the shortest possible time. Package plants typically use the activated sludge treatment method but other processes such as membrane reactors and biological aerated filters are also produced in a package plant format. It is very important when deciding on a package plant to ensure that a maintenance contract is in place with the supplier of the plant, as like all treatment processes, they do not run on their own entirely unattended. In some municipal areas, such as those in South Africa’s Kwazulu Natal province, it is a municipal requirement that a deposit covering five years or more of the operation of a package plant or 50% of its cost is held by the relevant municipal body to avoid the problem of failing plants that no-one takes responsibility for.

Anaerobic Digestors
As the name implies, this type of effluent treatment relies on the anaerobic conditions in that microorganisms break down biodegradable material in the absence of oxygen in either the free form or chemically bound as nitrates. A septic tank breaks down waste material mostly under anaerobic conditions. Anaerobic digestors are not suited to all waste types and can be unforgiving in that the process can be interrupted permanently should the digestor’s microorganisms experience a toxic shock resulting from other pollutants entering the waste stream. The anaerobic process also usually requires a constant feed of uniform quality waste to operate efficiently which places additional operation requirements on the process and is therefore not suited to waste that is seasonal. When batch-fed, i.e. intermittently feeding over a period of time, the anaerobic digestion process can produce very unpleasant odours. The effluent produced from an anaerobic digestor typically requires further aerobic treatment to reduce the concentration of organic contaminants and nutrients to acceptable levels before it can discharged to the environment, unless it is on a very small scale where it is possible to employ the use of a soakaway used in conjunction with a septic tank. Anaerobic digestion is also often used in a conventional activated sludge treatment works treating sewage to remove phosphorous as part of the nutrient removal process and waste sludge produced by such a plant is also sometimes digested anaerobically prior to disposal to a landfill.

A byproduct of the anaerobic breakdown of organic matter is biogas which consists mostly of methane (CH4: 50-75%) and Carbon Dioxide (CO2:25-50%) as well as trace amounts of other gases such as Nitrogen (N2), Hydrogen (H2) and Hydrogen Sulphide (H2S) and is therefore considered to be a greenhouse gas that should ideally be captured and burnt off. The methane in biogas is often separated from the other gases so that it can be used in industry for heating purposes. The first anaerobic digester was built by a leper colony in Bombay, India in 1859 and street lights were first lit using gas produced from a septic tank in Exeter, England in 1895 and currently there are over 4000 anaerobic digestors in Germany and 300 000 in China where they are used to develop methane gas for heating and electricity generation.

Although the batch method of supplying effluent to an anaerobic digestor is the simplest and least capital intensive method of this type of treatment with low levels of operator input and equipment, the attendant problems of odour production, likelihood of process failure due to inconsistency of the effluent and unsuitability to constant waste flows which would necessitate expensive and odorous storage, a continuous digestion process is usually preferable. There are two basic types of continuous processes used in wastewater treatment:

• Upflow Anaerobic Sludge Blanket (UASB)
• Expanded Granular Sludge Bed (EGSB)

 

UASB
The Upflow Anaerobic Sludge Blanket digestor relies on the wastewater effluent flowing upwards through a blanket of granular sludge suspended in the tank. The blanket is formed by small sludge granules comprising aggregates of bacteria. The wastewater stream leaving a UASB digestor requires further treatment using aerobic processes to reduce the organic contaminant levels but the biogas produced by the UASB process has a high percentage of methane. It is important to control the flow of effluent into a UASB digestor very carefully so as not to break up the sludge bed and therefore this type of treatment is not suited to all applications.

EGSB
A type of UASB process, in an Expanded Granular Sludge Bed digestor the upward flow rate of wastewater passing through the sludge blanket is increased so that the blanket becomes partially fluidized which improves sludge contact with the wastewater and enhances segregation of small inactive suspended particle from the sludge bed. The increased flow rate is typically brought about by recycling the sludge or using very tall reactor vessels.

Tertiary Treatment

The purpose of tertiary treatment, or polishing, is to improve the effluent quality before it is discharged back into the environment. This is typically achieved using either or reedbeds that have been constructed specifically for this purpose using suitable materials and plants.

Lagoons
The wastewater effluent discharged from a secondary treatment process is improved in a lagoon (or pond) by the action of settlement and biological improvement through the activity of other naturally occurring organisms, flora and fauna. Lagoons need to be highly aerobic and it is important to size the lagoons according to the effluent flow rate entering them to avoid them becoming overloaded and turning anaerobic.