Activated Sludge Magnified

ACTIVATED SLUDGE:  The activated sludge process is a process for treating sewage and industrial wastewaters using air and a biological floc composed of bacteria and protozoa.  Its important to realize that the name ‘Activated Sludge’ is reserved for aerobic processes, where in anaerobic processes also active bacteria are in function and mostly in very high volumetric rates.

AERATED LAGOON or AERATED BASIN:  An aerated lagoon or aerated basin is a holding and/or treatment pond provided with artificial aeration to promote the biological oxidation of wastewaters. There are many other biological processes for treatment of waste waters, for example activated sludgetrickling filtersrotating biological contactors and biofilters. They all have in common the use of oxygen (or air) and microbial action to bio-treat the pollutants in waste waters.

Oxygen Demand being Tested at WWTP

BIOCHEMICAL OXYGEN DEMAND (BOD):  Also called biological oxygen demand, is the amount of dissolved oxygen needed (i.e., demanded) by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 °C and is often used as a surrogate of the degree of organic pollution of water.  BOD can be used as a gauge of the effectiveness of wastewater treatment plants. It is listed as a conventional pollutant in the U.S. Clean Water Act.  BOD is similar in function to chemical oxygen demand (COD), in that both measure the amount of organic compounds in water. However, COD is less specific, since it measures everything that can be chemically oxidized, rather than just levels of biodegradable organic matter.

Pumpkin seedlings planted out on windrows of composted bio-solids

BIO-SOLIDS:  A term coined in the United States that is typically used to describe several forms of treated sewage sludge that is intended for agricultural use as a soil conditioner.  Although sewage sludge has long been used in agriculture, concerns about offensive odors and disease risks from pathogens and toxic chemicals may reduce public acceptance of the practice. Modern use of the term bio-solids may be subject to government regulations, although informal use describes a broader range of semi-solid organic products separated from sewage.  Description of bio-solids in conformance with local regulations may reduce confusion; but some use an expanded definition including any solids, slime solids or liquid slurry residue generated during the treatment of domestic sewage including scum and solids removed during primary, secondary or advanced treatment processes. Use of alternative terms like solids or wastewater solids may be preferable for non-conforming bio-solids.  Bio-solids may be defined as organic wastewater solids that can be reused after suitable sewage sludge treatment processes leading to sludge stabilization such as anaerobic digestion and composting. Alternatively, the bio-solids definition may be restricted by local regulations to wastewater solids only after those solids have completed a specified treatment sequence and/or have concentrations of pathogens and toxic chemicals below specified levels.  The United States Environmental Protection Agency (USEPA) defines the two terms – sewage sludge and bio-solids – in the Code of Federal Regulations (CFR), Title 40, Part 503 as follows: sewage sludge refers to the solids separated during the treatment of municipal wastewater (including domestic septage), while bio-solids refers to treated sewage sludge that meets the USEPA pollutant and pathogen requirements for land application and surface disposal.  A similar definition has been used internationally.

CARBON FILTERING:  A method of filtering that uses a bed of activated carbon to remove contaminants and impurities, using chemical adsorption.  Each particle/granule of carbon provides a large surface area/pore structure, allowing contaminants the maximum possible exposure to the active sites within the filter media. One pound (454 g) of activated carbon contains a surface area of approximately 100 acres (~40 Hectares).  Activated carbon works via a process called adsorption, whereby pollutant molecules in the fluid to be treated are trapped inside the pore structure of the carbon substrate. Carbon filtering is commonly used for water purification, in air purifiers and industrial gas processing, for example the removal of siloxanes and hydrogen sulfide from biogas. It is also used in a number of other applications, including respirator masks, the purification of sugarcane and in the recovery of precious metals, especially gold. It is also used in cigarette filters.  Active charcoal carbon filters are most effective at removing chlorine, sediment, volatile organic compounds (VOCs), taste and odor from water. They are not effective at removing minerals, salts, and dissolved inorganic compounds.  Typical particle sizes that can be removed by carbon filters range from 0.5 to 50 micrometres. The particle size will be used as part of the filter description. The efficacy of a carbon filter is also based upon the flow rate regulation. When the water is allowed to flow through the filter at a slower rate, the contaminants are exposed to the filter media for a longer amount of time.  There are two predominant types of carbon filters used in the filtration industry: powdered block filters and granular activated filters. In general, carbon block filters are more effective at removing a larger number of contaminants, based upon the increased surface area of carbon. Many carbon filters also use secondary media such as silver to prevent bacteria growth within the filter. Alternatively, the activated carbon itself may be impregnated with silver to provide this bacteriostatic property.

CARBONOCEOUS BIO-CHEMICAL OXYGEN DEMAND:   Referred to as CBOD, is a method defined test measured by the depletion of dissolved oxygen by biological organisms in a body of water in which the contribution from nitrogenous bacteria has been suppressed. CBOD is a method defined parameter is widely used as an indication of the pollutant removal from wastewater. It is listed as a conventional pollutant in the U.S. Clean Water Act.  The CBOD tests have the widest application in measuring waste loadings to treatment plants and in evaluating the CBOD-removal efficiency of such treatment systems. The test measures the molecular oxygen utilized during a specified incubation period for the biochemical degradation of organic material (carbonaceous demand) and the oxygen used to oxidize inorganic material such as sulfides and ferrous iron. It also may measure the amount of oxygen used to oxidize reduced forms of nitrogen (nitrogenous demand) unless their oxidation is prevented by an inhibitor. The seeding and dilution procedures provide an estimate of the CBOD at pH 6.5 to 7.5.

There are two recognized EPA methods for the measurement of CBOD:

  • Standard Methods for the Examination of Water and Wastewater, Method 5210B
  • In-Situ Inc. Method 1004-8-2009 Carbonaceous Biochemical Oxygen Demand (CBOD) Measurement by Optical Probe.

CHEMICAL OXYGEN DEMAND:  In environmental chemistry, the chemical oxygen demand (COD) is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of solution which in SI units is milligrams per litre(mg/L). A COD test can be used to easily quantify the amount of organics in water. The most common application of COD is in quantifying the amount of oxidizable pollutants found in surface water (e.g. lakes and rivers) or wastewater. COD is useful in terms of water quality by providing a metric to determine the effect an effluent will have on the receiving body much like biochemical oxygen demand (BOD).  In environmental chemistry, the chemical oxygen demand (COD) is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of solution which in SI units is milligrams per litre(mg/L). A COD test can be used to easily quantify the amount of organics in water. The most common application of COD is in quantifying the amount of oxidizable pollutants found in surface water (e.g. lakes and rivers) or wastewater. COD is useful in terms of water quality by providing a metric to determine the effect an effluent will have on the receiving body much like biochemical oxygen demand (BOD).

  Clarifier at Waste Water Treatment Plant

CLARIFIER:  Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and (or) thickening. Concentrated impurities, discharged from the bottom of the tank are known as sludge, while the particles that float to the surface of the liquid are called scum.  Before the water enters the clarifier, coagulation and flocculation reagents, such as polyelectrolytes and ferric sulfate, can be added. These reagents cause finely suspended particles to clump together and form larger and denser particles, called flocs, that settle more quickly and stably. This allows the separation of the solids in the clarifier to occur more efficiently and easily; aiding in the conservation of energy. Isolating the particle components first using these processes may reduce the volume of downstream water treatment processes like filtration.  Water being purified for human consumption, is treated with floculation reagents, then sent to the clarifier where removal of the flocculated coagulate occurs producing clarified water. The clarifier works by permitting the heavier and larger particles to settle to the bottom of the clarifier. The particles then form a bottom layer of sludge requiring regular removal and disposal. Clarified water then proceeds through several more steps before being sent for storage and use.

COLIFORM INDEX:  The coliform index is a rating of the purity of water based on a count of fecal bacteria. It is one of many tests done to assure sufficient water quality.  Coliform bacteria are microorganisms that primarily originate in the intestines of warm-blooded animals. By testing for coliforms, especially the well known Escherichia coli (E. coli), which is a thermotolerant coliform, one can determine if the water has possibly been exposed to fecal contamination; that is, whether it has come in contact with human or animal feces. It is important to know this because many disease-causing organisms are transferred from human and animal feces to water, from where they can be ingested by people and infect them. Water that has been contaminated by feces usually contains pathogenic bacteria, which can cause disease. Some types of coliforms cause disease, but the coliform index is primarily used to judge if other types of pathogenic bacteria are likely to be present in the water.

The coliform index is used because it is difficult to test for pathogenic bacteria directly. There are many different types of disease-causing bacteria, and they are usually present in low numbers which do not always show up in tests. Thermotolerant coliforms are present in higher numbers than individual types of pathogenic bacteria and they can be tested relatively easily.

However, the coliform index is far from perfect. Thermotolerant coliforms can survive in water on their own, especially in tropical regions, so they do not always indicate fecal contamination. Furthermore, they do not give a good indication of how many pathogenic bacteria are present in the water, and they give no idea at all of whether there are pathogenic viruses or protozoa which also cause diseases and are rarely tested for. Therefore, it does not always give accurate or useful results regarding the purity of water.

GREEN HOUSE GAS:  A greenhouse gas (abbrev. GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect.  The primary greenhouse gases in Earth’s atmosphere are water vaporcarbon dioxidemethanenitrous oxide, and ozone. Without greenhouse gases, the average temperature of Earth’s surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F).  In the Solar System, the atmospheres of VenusMars and Titan also contain gases that cause a greenhouse effect.  Human activities since the beginning of the Industrial Revolution (taken as the year 1750) have produced a 40% increase in the atmospheric concentration of carbon dioxide, from 280 ppm in 1750 to 406 ppm in early 2017.  This increase has occurred despite the uptake of a large portion of the emissions by various natural “sinks” involved in the carbon cycle.  Anthropogenic carbon dioxide (CO2) emissions (i.e., emissions produced by human (activities) come from combustion of fossil fuels, principally coaloil, and natural gas, along with deforestation, soil erosion and animal agriculture.  It has been estimated that if greenhouse gas emissions continue at the present rate, Earth’s surface temperature could exceed historical values as early as 2047, with potentially harmful effects on ecosystems, biodiversity and the livelihoods of people worldwide.  Recent estimates suggest that on the current emissions trajectory the Earth could pass a threshold of 2 °C global warming, which the United Nations’ IPCC designated as the upper limit to avoid “dangerous” global warming, by 2036.

MEMBRANE BIOREACTOR (MBR):  A membrane is the combination of a membrane process like microfiltration or ultra filtration with a suspended growth  bioreactor, and is now widely used for municipal and industrial wastewater treatment with plant sizes up to 80,000 population equivalent (i.e. 48 million liters per day).  When used with domestic wastewater, MBR processes can produce effluent of high quality enough to be discharged to coastal, surface or brackish waterways or to be reclaimed for urban irrigation. Other advantages of MBRs over conventional processes include small footprint, easy retrofit and upgrade of old wastewater treatment plants.  It is possible to operate MBR processes at higher mixed liquor suspended solids (MLSS) concentrations compared to conventional settlement separation systems, thus reducing the reactor volume to achieve the same loading rate.  Two MBR configurations exist: internal/submerged, where the membranes are immersed in and integral to the biological reactor; and external/side stream, where membranes are a separate unit process requiring an intermediate pumping step.  Recent technical innovation and significant membrane cost reduction have enabled MBRs to become an established process option to treat waste waters.  As a result, the MBR process has now become an attractive option for the treatment and reuse of industrial and municipal waste waters, as evidenced by their constantly rising numbers and capacity. The current MBR market has been estimated to value around US$216 million in 2006 and to rise to US$363 million by 2010.

MIXED LIQUOR SUSPENDED SOLIDS:  Mixed liquor suspended solids (MLSS) is the concentration of suspended solids, in an aeration tank during the activated sludge process, which occurs during the treatment of waste water. The units MLSS is primarily measured in are milligram per litre (mg/L), but for activated sludge its mostly measured in gram per liter [g/l] which is equal to kilogram per m3 [kg/m3]. Mixed liquor is a combination of raw or unsettled wastewater or pre-settled wastewater and activated sludge within an aeration tank. MLSS consists mostly of microorganisms and non-biodegradable suspended matter. MLSS is an important part of the activated sludge process to ensure that there is a sufficient quantity of active biomass available to consume the applied quantity of organic pollutant at any time. This is known as the food to microorganism ratio, more commonly notated as the F/M ratio. By maintaining this ratio at the appropriate level the biomass will consume high percentages of the food. This minimizes the loss of residual food in the treated effluent. In simple terms, the more the biomass consumes the lower the biochemical oxygen demand (BOD) will be in the discharge. It is important that MLSS removes COD and BOD in order to purify water for clean surface waters, and subsequently clean drinking water and hygiene. Raw sewage enters in the water treatment process with a concentration of sometimes several hundred mg/L of BOD. Upon being treated with MLSS and other methods of treatment, the concentration of BOD in water is lowered to less than 2 mg/L, which is considered to be clean, safe to discharge to surface waters or to reuse water.  The total weight of MLSS within an aeration tank can be calculated by multiplying the concentration of MLSS (kg/m3) in the aeration tank by the tank volume (m3).

MLSS is responsible for removing the biochemical oxygen demand make-up of a large portion of the solids that are retained in the activated sludge process within the water treatment process. They are the “active” part of activated sludge process. Mixed liquor suspended solids are the solids under aeration. MLSS is measured by filtering a known volume of the mixed liquor sample, which is the same way that suspended solids are measured in wastewater. Some of the MLSS may be inorganic material. Sometimes this may represent a large percentage of the solids present in the wastewater.

Environmental engineering focuses on the particles suspended in water and the suitable operation of water treatment plants. Therefore, it is important to measure the total mass of suspended solids, which is the MLSS, as well as the mass of organic matter suspended in the activated sludge unit. These measurements allow engineers to adjust the flow rate of return sludge from the secondary clarifier into the secondary treatment reactor. This ensures that influent organic matter will be treated with a correct concentration of microorganisms.

OIL WATER SEPARATOR:  An API oil–water separator is a device designed to separate gross amounts of oil and suspended solids from the wastewater effluents of oil refineriespetrochemical plantschemical plantsnatural gas processing plants and other industrial oily water sources. The name is derived from the fact that such separators are designed according to standards published by the American Petroleum Institute (API).  The API separator is a gravity separation device designed using Stokes’ law principles that define the rise velocity of oil droplets based on their density, size and water properties. The design of the separator is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. Based on that design criterion, most of the suspended solids will settle to the bottom of the separator as a sediment layer, the oil will rise to top of the separator, and the wastewater will be the middle layer between the oil on top and the solids on the bottom.  The API Design Standards, when correctly applied, make adjustments to the geometry, design and size of the separator beyond simple Stokes Law principles. This includes allowances for water flow entrance and exit turbulence losses as well as other factors.  Typically in operation of API separators the oil layer, which may contain en-trained water and attached suspended solids, is continually skimmed off. This removed oily layer may be re-processing to recover valuable products, or disposed of. The heavier bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump.

       Example of Reuse or Treated Water


OXYGEN SATURATION:  Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water. The standard unit of oxygen saturation is percent (%).  Oxygen saturation can be measured regionally and noninvasively.   Arterial oxygen saturation (SaO2) is commonly measured using pulse oximetry. Tissue saturation at peripheral scale can be measured using NIRS. This technique can be applied on both muscle and brain.  In aquatic environments, oxygen saturation is a ratio of the concentration of dissolved oxygen (O2) in the water to the maximum amount of oxygen that will dissolve in the water at that temperature and pressure under stable equilibrium. Well-aerated water (such as a fast-moving stream) without oxygen producers or consumers is 100% saturated.  It is possible for stagnant water to become somewhat supersaturated with oxygen (i.e. reach more than 100% saturation) either because of the presence of photosynthetic aquatic oxygen producers or because of a slow equilibration after a change of atmospheric conditions.  Stagnant water in the presence of decaying matter will typically have an oxygen concentration much less than 100%.  Environmental oxygenation can be important to the sustainability of a particular ecosystem. Refer to ([1] for a table of maximum equilibrium dissolved oxygen concentration versus temperature at atmospheric pressure. The optimal levels in an estuary for dissolved oxygen is higher than 6 ppm.[citation needed] Insufficient oxygen (environmental hypoxia), often caused by the decomposition of organic matter and/or nutrient pollution, may occur in bodies of water such as ponds and rivers, tending to suppress the presence of aerobic organisms such as fish.  Deoxygenation increases the relative population of anaerobic organisms such as plants and some bacteria, resulting in fish kills and other adverse events. The net effect is to alter the balance of nature by increasing the concentration of anaerobic over aerobic species.

RECLAIMED WATER:  Treated wastewater can be reused in industry (for example in cooling towers), in artificial recharge of aquifers, in agriculture and in the rehabilitation of natural ecosystems (for example in Florida’s Everglades). In rarer cases it is also used to augment drinking water supplies.  There are several technologies used to treat wastewater for reuse. A combination of these technologies can meet strict treatment standards and make sure that the processed water is hygienically safe, meaning free from bacteria and viruses. The following are some of the typical technologies: Ozonationultra filtrationaerobic treatment (membrane bioreactor), forward osmosisreverse osmosisadvanced oxidation.  Some water demanding activities do not require high grade water. In this case, wastewater can be reused with little or no treatment. One example of this scenario is in the domestic environment where toilets can be flushed using greywater from baths and showers with little or no treatment.

  • Gardening and agriculture: There are benefits of using recycled water for irrigation, including the lower cost compared to some other sources and consistency of supply regardless of season, climatic conditions and associated water restrictions. Irrigation with recycled wastewater can also serve to fertilize plants if it contains nutrients, such as nitrogen, phosphorus and potassium.  In developing countries, agriculture is increasingly using untreated wastewater for irrigation. Cities provide lucrative markets for fresh produce, so are attractive to farmers. However, because agriculture has to compete for increasingly scarce water resources with industry and municipal users, there is often no alternative for farmers but to use water polluted with urban waste directly to water their crops.
  • Health risks:  There can be significant health hazards related to using untreated wastewater in agriculture. Wastewater from cities can contain a mixture of chemical and biological pollutants. In low-income countries, there are often high levels of pathogens from excreta, while in emerging nations, where industrial development is outpacing environmental regulation, there are increasing risks from inorganic and organic chemicals. The World Health Organization, in collaboration with the Food and Agriculture Organization of the United Nations (FAO) and the United Nations Environmental Program (UNEP), has developed guidelines for safe use of wastewater in 2006.  These guidelines advocate a ‘multiple-barrier’ approach to wastewater use, for example by encouraging farmers to adopt various risk-reducing behaviours. These include ceasing irrigation a few days before harvesting to allow pathogens to die off in the sunlight, applying water carefully so it does not contaminate leaves likely to be eaten raw, cleaning vegetables with disinfectant or allowing fecal sludge used in farming to dry before being used as a human manure.

ROTATIONAL BIOLOGICAL CONTACTOR (RBC):  An RBC or rotating biological contactor is a biological treatment process used in the treatment of wastewater following primary treatment.  The primary treatment process means protection by removal of grit and sand and coarse material through a screening process, followed by a removal process of sediment by settling. The RBC process involves allowing the wastewater to come in contact with a biological medium in order to remove pollutants in the wastewater before discharge of the treated wastewater to the environment, usually a body of water (river, lake or ocean). A rotating biological contactor is a type of secondary (Biological) treatment process. It consists of a series of closely spaced, parallel discs mounted on a rotating shaft which is supported just above the surface of the waste water. Microorganisms grow on the surface of the discs where biological degradation of the wastewater pollutants takes place.

The rotating packs of disks (known as the media) are contained in a tank or trough and rotate at between 2 and 5 revolutions per minute. Commonly used plastics for the media are polyethylene, PVC expanded polystyrene. The shaft is aligned with the flow of wastewater so that the discs rotate at right angles to the flow, with several packs usually combined to make up a treatment train. About 40% of the disc area is immersed in the wastewater.

Biological growth is attached to the surface of the disc and forms a slime layer. The discs contact the wastewater with the atmospheric air for oxidation as it rotates. The rotation helps to slough off excess solids. The disc system can be staged in series to obtain nearly any detention time or degree of removal required. Since the systems are staged, the culture of the later stages can be acclimated to the slowly degraded materials.

The discs consist of plastic sheets ranging from 2 to 4 m in diameter and are up to 10 mm thick. Several modules may be arranged in parallel and/or in series to meet the flow and treatment requirements. The discs are submerged in waste water to about 40% of their diameter. Approximately 95% of the surface area is thus alternately submerged in waste water and then exposed to the atmosphere above the liquid. Carbonaceous substrate is removed in the initial stage of RBC. Carbon conversion may be completed in the first stage of a series of modules, with nitrification being completed after the 5th stage. Most design of RBC systems will include a minimum of 4 or 5 modules in series to obtain nitrification of waste water.

Biofilm, which are biological growths that become attached to the discs, assimilate the organic materials in the wastewater. Aeration is provided by the rotating action, which exposes the media to the air after contacting them with the wastewater, facilitating the degradation of the pollutants being removed. The degree of wastewater treatment is related to the amount of media surface area and the quality and volume of the in-flowing wastewater.

SEPTIC FIELD:  Septic fields, also called septic drain fields,  leach fields or leach drains, are subsurface wastewater disposal facilities used to remove contaminants and impurities from the liquid that emerges after anaerobic digestion in a septic tank.  A septic tank, the septic drain field, and the associated piping compose a septic system. The septic drain field is effective for disposal of organic materials  readily catabolized by a microbial ecosystem.  The drain field typically consists of an arrangement of trenches containing perforated pipes and porous material (often gravel) covered by a layer of soil to prevent animals (and surface runoff) from reaching the wastewater distributed within those trenches.  Primary design considerations are hydraulic for the volume of wastewater requiring disposal and catabolic for the long-term biochemical oxygen demand of that wastewater.  Sewage farms are similarly used to dispose of wastewater through a series of ditches and lagoons (often with little or no pre-treatment). These are more often found in arid countries as the water flow on the surface allows for irrigation (and fertilization) of agricultural land.  Many health departments require a percolation test (“perc” test) to establish suitability of drain field soil to receive septic tank effluent.  An engineer or licensed designer may be required to work with the local governing agency to design a system that conforms to these criteria.

Wastewater from toilets is assumed to contain bacteria and viruses capable of causing disease. Disinfection methods used prior to surface disposal of municipal sewage cannot be used with septic tanks because disinfection would prevent wastewater treatment by killing the septic tank and soil ecosystems catabolizing the putrescible contents of the wastewater. A properly functioning drain field holds and deactivates pathogens before they leave the drain field soil.

The goal of percolation testing is to ensure the soil is permeable enough for septic tank effluent to percolate away from the drain field, but fine grained enough to filter out pathogenic bacteria and viruses before they travel far enough to reach a water well or surface water supply. Coarse soils – sand and gravel – can transmit wastewater away from the drain field before pathogens are destroyed. Silt and clay effectively filter out pathogens but allow very limited wastewater flow rates.[2] Percolation tests measure the rate at which clean water disperses through a disposal trench into the soil. Several factors may reduce observed percolation rates when the drain field receives anoxic septic tank effluent:[3]

  • Microbial colonies catabolizing soluble organic compounds from the septic tank effluent will adhere to soil particles and reduce the interstitial area available for water flow between soil particles. These colonies tend to form a low-permeabilitybiofilm of gelatinous slime at the soil interface of the disposal trench.[4]
  • Insoluble particles small enough to be carried through the septic tank will accumulate at the soil interface of the disposal trench; non-biodegradable particles likesynthetic fiber lint from laundry, mineral soil from washing, or bone and eggshell fragments from garbage disposals will remain to fill interstitial areas formerly available for water flow out of the trench.[5]
  • Cooking fats orpetroleum products emulsified by detergents or dissolved by solvents can flow through prior to anaerobic liquefaction when septic tank volume is too small to offer adequate residence time, and may congeal as a hydrophobic layer on the soil interface of the disposal trench.[6]
  • Risinggroundwater levels may reduce the available hydraulic head (or vertical distance) causing gravitational water flow away from the disposal trench. Effluent initially flowing downward from the disposal trench ultimately encounters groundwater or impermeable rock or clay requiring a directional shift to horizontal movement away from the drain field. A certain vertical distance is required between the effluent level in the disposal trench and the water level where the effluent is leaving the drain field for gravitational force to overcome viscous frictional forces resisting flow through porous soil. Effluent levels in the vicinity of the drain field will appear to rise toward the ground surface to preserve that vertical distance difference if groundwater levels surrounding the drain field approach the level of effluent in the disposal trench.[6]
  • Frozen ground may seasonally reduce the cross-sectional area available for flow or evaporation.

SEPTIC TANK:  A septic tank is a watertight chamber made of concrete, fibreglass, PVC or plastic, through which domestic wastewater (sewage) flows for primary treatment.  Settling and anaerobic processes reduce solids and organics, but the treatment is only moderate.  Septic tank systems are a type of onsite sewage facility (OSSF). They can be used in areas that are not connected to a sewerage system, such as rural areas.  The treated liquid effluent is commonly disposed in a septic drain field which provides further treatment.  However, groundwater pollution may occur and can be a problem.  The term “septic” refers to the anaerobic bacterial environment that develops in the tank which decomposes or mineralizes the waste discharged into the tank.  Septic tanks can be coupled with other onsite wastewater treatment units such as biofilters or aerobic systems involving artificially forced aeration.  The rate of accumulation is sludge – also called septage or fecal sludge – is faster than the rate of decomposition.  Therefore, the accumulated fecal sludge must be periodically removed which is commonly done with a vacuum truck.

A septic tank consists of one or more concrete or plastic tanks of between 4000 and 7500 liters (1,000 and 2,000 gallons); one end is connected to an inlet wastewater pipe and the other to a septic drain field. Generally these pipe connections are made with a T pipe, allowing liquid to enter and exit without disturbing any crust on the surface. Today, the design of the tank usually incorporates two chambers, each equipped with a manhole cover, and separated by a dividing wall with openings located about midway between the floor and roof of the tank.  Wastewater enters the first chamber of the tank, allowing solids to settle and scum to float. The settled solids are anaerobically digested, reducing the volume of solids. The liquid component flows through the dividing wall into the second chamber, where further settlement takes place. The excess liquid, now in a relatively clear condition, then drains from the outlet into the septic drain field, also referred to as a leach field, drain field or seepage field, depending upon locality. A percolation test is required prior to installation to ensure the porosity of the soil is adequate to serve as a drain field.  

The remaining impurities are trapped and eliminated in the soil, with the excess water eliminated through percolation into the soil, through evaporation, and by uptake through the root system of plants and eventual transpiration or entering groundwater or surface water. A piping network, often laid in a stone-filled trench (see weeping tile), distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the drain field is proportional to the volume of wastewater and inversely proportional to the porosity of the drainage field. The entire septic system can operate by gravity alone or, where topographic considerations require, with inclusion of a lift pump. Certain septic tank designs include siphons or other devices to increase the volume and velocity of outflow to the drainage field. These help to fill the drainage pipe more evenly and extend the drainage field life by preventing premature clogging or bioclogging.

A properly designed and normally operating septic system is odor-free and, besides periodic inspection and emptying of the septic tank, should last for decades with minimal maintenance.  A well designed and maintained concrete, fiberglass, or plastic tank should last about 50 years.

Waste that is not decomposed by the anaerobic digestion must eventually be removed from the septic tank. Otherwise the septic tank fills up and wastewater containing undecomposed material discharges directly to the drainage field. Not only is this detrimental for the environment but, if the sludge overflows the septic tank into the leach field, it may clog the leach field piping or decrease the soil porosity itself, requiring expensive repairs.

When a septic tank is emptied, the accumulated sludge (septage, also known as fecal sludge) is pumped out of the tank by a vacuum truck. How often the septic tank must be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids, and the ambient temperature (because anaerobic digestion occurs more efficiently at higher temperatures), as well as usage, system characteristics and the requirements of the relevant authority. Some health authorities require tanks to be emptied at prescribed intervals, while others leave it up to the decision of an inspector. Some systems require pumping every few years or sooner, while others may be able to go 10–20 years between pumpings. An older system with an undersized tank that is being used by a large family will require much more frequent pumping than a new system used by only a few people. Anaerobic decomposition is rapidly restarted when the tank is refilled.

SEPTIC TANK MAINTENANCE:  Like any system, a septic system requires maintenance. The maintenance of a septic system is often the responsibility of the resident or property owner. Some forms of abuse or neglect include the following:

User’s actions;

  • Excessive disposal of cooking oils and grease can cause the inlet drains to block. Oils and grease are often difficult to degrade and can cause odor problems and difficulties with the periodic emptying.
  • Flushing non-biodegradable waste items down the toilet such ascigarette buttscotton buds/swabs or menstrual hygiene products (e.g. sanitary napkins or tampons) and condoms can cause a septic tank to clog and fill rapidly. Therefore, these materials should not be disposed of in that manner; the same applies when the toilet is connected to a sanitary sewer instead of a septic tank.
  • Using the toilet for disposal of food waste can cause a rapid overload of the system with solids and contribute to failure.[8]
  • Certain chemicals may damage the components of a septic tank or kill the bacteria needed in the septic tank for the system to operate properly, such as pesticidesherbicides, materials with high concentrations of bleach or caustic soda (lye), or any other inorganic materials such as paints or solvents.
  • The flushing of salted water into the septic system can lead to sodium binding in the drainfield. The clay and fine silt particles bind together and effectively waterproof the leach field, rendering it ineffective.

Other factors;

  • Roots from trees and shrubbery protruding above the tank or drainfield may clog and/or rupture them. Trees that are directly within the vicinity of a concrete septic tank have the potential to penetrate the tank as the system ages and the concrete begins to develop cracks and small leaks. Tree roots can cause serious flow problems due to plugging and blockage of drain pipes, added to which the trees themselves tend to expand extremely vigorously due to the ready supply of nutrients from the septic system.
  • Playgrounds and storage buildings may cause damage to a tank and the drainage field. In addition, covering the drainage field with an impermeable surface, such as a driveway or parking area, will seriously affect its efficiency and possibly damage the tank and absorption system.
  • Excessive water entering the system may overload it and cause it to fail.
  • Very high rainfall, rapidsnowmelt, and flooding from rivers or the sea can all prevent a drain field from operating, and can cause flow to back up, interfering with the normal operation of the tank. High winter water tables can also result in groundwater flowing back into the septic tank.
  • Over time,biofilms develop on the pipes of the drainage field, which can lead to blockage. Such a failure can be referred to as “biomat failure”.

Septic tank additives;

Septic tank additives have been promoted by some manufacturers with the aim to improve the effluent quality from septic tanks, reduce sludge build-up and to reduce odors. However, these additives – which are commonly based on “effective microorganisms” – are usually costly in the longer term and fail to live up to expectations. It has been estimated that in the U.S. more than 1,200 septic system additives were available on the market in 2011.  However, very little peer-reviewed and replicated field research exists regarding the efficacy of these biological septic tank additives.

Simplified process flow diagram for a typical large-scale treatment plant

SEWAGE TREATMENT:  Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage. It includes physical, chemical, and biological processes to remove these contaminants and produce environmentally safer treated wastewater (or treated effluent). A by-product of sewage treatment is usually a semi-solid waste or slurry, called sewage sludge,that has to undergo further treatment before being suitable for disposal or land application.  Sewage treatment may also be referred to as wastewater treatment, although the latter is a broader term which can also be applied to purely industrial wastewater. For most cities, the sewer system will also carry a proportion of industrial effluent to the sewage treatment plant which has usually received pretreatment at the factories themselves to reduce the pollutant load. If the sewer system is a combined sewer then it will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage water can travel towards treatment plants via piping and in a flow aided by gravity and pumps. The first part of filtration of sewage typically includes a bar screen to filter solids and large objects which are then collected in dumpsters and disposed of in landfills. Fat and grease will also be removed before the primary treatment of sewage.

WASTEWATER:  Wastewater, also written as waste water, is any water that has been adversely affected in quality by anthropogenic influence. Wastewater can originate from a combination of domestic, industrial, commercial or agricultural activities, surface runoff or storm water, and from sewer inflow or infiltration.  Municipal wastewater (also called sewage) is usually conveyed in a combined sewer or sanitary sewer, and treated at a wastewater treatment plant. Treated wastewater is discharged into receiving water via an effluent pipe.  Wastewaters’ generated in areas without access to centralized sewer systems rely on on-site wastewater systems. These typically comprise a septic tankdrain field, and optionally an on-site treatment unit. The management of wastewater belongs to the overarching term sanitation, just like the management of human excretasolid waste and stormwater (drainage).

Sewage is a type of wastewater that comprises domestic wastewater and is therefore contaminated with feces or urine from people’s toilets, but the term sewage is also used to mean any type of wastewater. Sewerage is the physical infrastructure, including pipes, pumps, screens, channels etc. used to convey sewage from its origin to the point of eventual treatment or disposal.

Wastewater can come from:

Wastewater can be diluted or mixed with other types of water in the form of:

After it has undergone some treatment, the “treated wastewater” remains, e.g.:

The composition of wastewater varies widely. This is a partial list of what it may contain:

WATER CHLORINATION:  Water chlorination is the process of adding chlorine (Cl2) or hypochlorite to water. This method is used to kill certain bacteria and other microbes in tap water as chlorine is highly toxic. In particular, chlorination is used to prevent the spread of waterborne diseases such as choleradysenterytyphoid etc.  In a paper published in 1894, it was formally proposed to add chlorine to water to render it “germ-free”. Two other authorities endorsed this proposal and published it in many other papers in 1895. Early attempts at implementing water chlorination at a water treatment plant were made in 1893 in Hamburg, Germany, and in 1897 the town of Maidstone, England was the first to have its entire water supply treated with chlorine.

Permanent water chlorination began in 1905, when a faulty slow sand filter and a contaminated water supply caused a serious typhoid fever epidemic in Lincoln, England. Dr. Alexander Cruickshank Houston used chlorination of the water to stop the epidemic. His installation fed a concentrated solution of so-called chloride of lime to the water being treated. This was not simply modern calcium chloride, but contained chlorine gas dissolved in lime-water (dilute calcium hydroxide) to form calcium hypochlorite (chlorinated lime). The chlorination of the water supply helped stop the epidemic and as a precaution, the chlorination was continued until 1911 when a new water supply was instituted.

The first continuous use of chlorine in the United States for disinfection took place in 1908 at Boonton Reservoir (on the Rockaway River), which served as the supply for Jersey City, New Jersey.[5] Chlorination was achieved by controlled additions of dilute solutions of chloride of lime (calcium hypochlorite) at doses of 0.2 to 0.35 ppm. The treatment process was conceived by Dr. John L. Leal, and the chlorination plant was designed by George Warren Fuller. Over the next few years, chlorine disinfection using chloride of lime (calcium hypochlorite) were rapidly installed in drinking water systems around the world.[7]

The technique of purification of drinking water by use of compressed liquefied chlorine gas was developed by a British officer in the Indian Medical Service, Vincent B. Nesfield, in 1903. According to his own account, “It occurred to me that chlorine gas might be found satisfactory … if suitable means could be found for using it…. The next important question was how to render the gas portable. This might be accomplished in two ways: By liquefying it, and storing it in lead-lined iron vessels, having a jet with a very fine capillary canal, and fitted with a tap or a screw cap. The tap is turned on, and the cylinder placed in the amount of water required. The chlorine bubbles out, and in ten to fifteen minutes the water is absolutely safe. This method would be of use on a large scale, as for service water carts.”