Chemical Waste Gases Treatment

With chemical absorption, an ion, atom or molecule is absorbed into the free volume of the absorbing phase. The process of gas scrubbing is used, among other things, for the treatment of industrial waste gases or for the removal of odours. As an absorbent, water is used and – depending on the pollutant – the addition of specific chemicals (chemisorption) takes place. In most cases, very high treatment performance is achieved.

To achieve the required treatment performance at low operating costs. We designs and manufactures various kinds of scrubbing types for different applications such as

  • Counter-flow scrubber
  • Cross-flow scrubber
  • Direct-flow scrubber

They can be installed in one or more stages or can be used as emergency units.

In addition to scrubbers, We offers activated carbon filters for the adsorption of air pollutants. Activated carbon is fine-grained carbon with a highly porous structure and a very large surface. By attachment on the inner surface of the activated carbon, harmful or odorous substances are removed from gases, vapours and liquids.

In activated carbon filters, They can be used to eliminate a wide range of harmful or odorous substances such as

  • Hydrogen sulphide
  • Indole and skatole
  • Methylmercaptane
  • Methylamines
  • Ammonia
  • VOC

Waste Gas Treatment — Pyrolysis

The process waste gases are burnt in a decomposition zone. If required, a fuel gas can be applied. Depending on the chemical composition of the waste gases, various reactions take place, such as oxidation, reduction or pyrolysis.

 

 

 

Wet Scrubber Treating Water-Soluble Substances

Wet scrubbing is an efficient process for the treatment of water-soluble pollutants in process exhaust gases. Our wet scrubbers are employed mainly in the semiconductor industry wet treatment system. Installed closely behind the vacuum pumps, they optimally protect the fab’s exhaust gas system from contamination and corrosion. Maintenance is quick and easy.

Biologycal Waste Gases Treatment

In biofilters, harmful and odorous substances from waste-gas and exhaust-gas flows are decomposed to non-toxic, odourless and largely low molecular weight substances, such as carbon dioxide and water. This is done biochemically by the metabolic activity of microorganisms. For this purpose, the pollutant to be separated is first dissolved and microbiologically degraded.

It must be ensured that the metabolic activity of the microorganisms used can proceed at an optimum level. It is often useful here to use other substances such as, for example, phosphorus or trace elements.

We offers appropriate system types and construction tailored to the application

  • Bio scrubbers, biofilters, biotrickling filters/ trickle bed reactors
  • Single-stage or multi-stage – for example with upstream bio- or chemo-scrubbers
  • Surface, circular and floor filters
  • Container plants as well as compact and mobile biofilter plants
  • Closed biofilter for targeted waste-gas flow guidance

In biofilters, microorganisms are settled on an organic substrate (e.g. bark or woodchips). In addition to the pollutants, this serves the organisms as a nutrient substrate. For pollutant removal, the waste gas is blown into the filter bed from the bottom to the top. With their simple design, biofilters can be operated cost-effectively in applications with low pollutant loading.

In contrast to biofilters, the microorganisms used are suspended in the scrubbing liquid in bio scrubbers. The pollutants are extracted from the waste gas and then biodegraded. Depending on the pollutant load, the decomposition occurs in the absorber sump or in an additional external regeneration reactor.

In biotrickling filters, inert substrates are used instead of the biological substrates in conventional biofilter methods, on which the microorganisms are settled. Organic and inorganic gaseous compounds are biochemically oxidised under aerobic conditions and converted into low molecular weight, non-harmful compounds which are no longer perceptible, such as carbon dioxide and water. The laden waste gas flows through the substrate in cross, counter and direct flow with respect to the trickling liquid.

Electrostatic Collection of Micro and Nanoparticles

Electrostatic filters use an electric field to clean process waste gases from particles. Our systems filter micro and nanoparticle-containing or aerosol-containing process waste gases as they occur.

For the treatment of condensable organic compounds, we offer an electrostatic condensate separator.

Non Hazardous Waste

Industrial Materials

Common industrial materials that are considered non hazardous in most states include ash, sludges, antifreeze, grinding dusts and liquids contaminated with non-hazardous chemicals.

One material that differs in designation from state to state is asbestos. While in some states, asbestos is considered non hazardous, in the state of California, asbestos is considered to be hazardous.

Medical Waste

Many types of medical waste are considered hazardous. However, the majority of medical waste (85%) is not classified as hazardous.

Examples of non hazardous medical waste include plastic packaging, clean glass and plastic, paper and cardboard, and office products. Many medical products and treatments are stored in aerosol cans. In California, aerosol cans are not considered hazardous waste as long as they are completely depleted.

Electronic Waste

California has some of the most strict e-waste laws on the books. However, there are some forms of electronic waste that continue to be classified as non hazardous.

For example, the aluminum that is often found in electronic goods is considered to be non hazardous, as are copper and gold … two materials often found in computer parts. Plastic and glass parts are generally not hazardous as well.

However, because many of the examples above refer to parts, and not the electronic device as a whole, it can be difficult and time consuming to separate the non hazardous parts from the hazardous parts. It is usually better to simply recycle electronic waste at a facility that will accept your equipment.

 Identifying Non Hazardous Waste

Correctly identifying whether a waste is hazardous or non hazardous is one of the most important aspects of ensuring your company is meeting all local, state and federal regulations.

After all, before you can determine how to store and dispose of a waste, proper identification is necessary.

While some non hazardous waste is easier to identify than others, lab chemical waste and medical waste can be more difficult. Especially when working with waste such as this, it is critical to partner with a reputable disposal company that will identify wastes through sampling and testing so you can be certain the waste you generate is taken care of properly.

Once a waste is identified, the disposal company can then certify that waste as non hazardous, reducing your risk of running into trouble with authorities during the disposal process.

The best disposal companies will be able to assist you in differentiating between non hazardous waste and non-RCRA hazardous waste. RCRA, or the Resource Conservation and Recovery Act, is a federal public law that oversees the management of both hazardous and non hazardous waste.

While one type of waste may be considered non-RCRA hazardous waste, that waste may be considered hazardous waste under state laws. For example, the only type of e-waste that is restricted under the RCRA is the disposal of cathode ray tubes (CRTs), which are typically found in TVs and computer monitors.

Disposal Of Non Hazardous Waste

Although you have identified your waste as non hazardous, proper disposal of that waste is just as important, even if it has not been deemed an immediate public threat. For example, any type of waste can be harmful to the environment through other means, such as methane emissions during decomposition.

Non hazardous waste can typically be disposed of three different ways.

  1. Taken To A Disposal Site

Depending on the amount of non hazardous waste you need to dispose of, you may be able to simply take it to a Treatment, Storage and Disposal Facility (TSDF) or landfill. However, check with the disposal facility first to determine whether it accepts your waste and the quantity allowed.

  1. Recycle Your Waste

Depending on the type of non-hazardous waste your facility produces, a recycling facility may accept it. CalRecycle, California’s recycling and waste management program, oversees the recycling of several types of waste that businesses and facilities produce. This includes materials such as electronic waste, paint, organic material and beverage containers.

  1. Work With A Disposal Company

As mentioned above, working with a reputable non hazardous waste disposal company can provide many benefits, from waste identification to waste certification. A waste disposal company will also ensure your waste is properly disposed of according to all appropriate regulations.

For those who routinely generate non hazardous waste, working with a reputable disposal company can provide peace of mind that all cradle to grave requirements are being met as well. These specific regulations state that your company is responsible for all waste from the moment it is generated to the moment it is disposed of.

This places the burden on you to ensure your waste does not pose a threat to the community, and a certified non hazardous waste disposal company can work with you to make sure your company meets those requirements so that you can focus on the day-to-day operations of your business.

 

Landfill Gas

Landfill gas is a mix of different gases created by the action of microorganisms within a landfill as they decompose organic waste, including for example, food waste and paper waste. Landfill gas is approximately forty to sixty percent methane, with the remainder being mostly carbon dioxide. Trace amounts of other volatile organic compounds (VOCs) comprise the remainder (<1%). These trace gases include a large array of species, mainly simple hydrocarbons.

Landfill gases have an influence on climate change. The major components are CO2 and methane, both of which are greenhouse gasses. Methane in the atmosphere is a far more potent greenhouse gas, with each molecule having twenty-five times the effect of a molecule of carbon dioxide. Methane itself however accounts for less composition of the atmosphere than does carbon dioxide. Landfills are the third-largest source of methane in the US.

Because of the significant negative effects of these gases, regulatory regimes have been set up to monitor landfill gas, reduce the amount of biodegradable content in municipal waste, and to create landfill gas utilization strategies, which include gas flaring or capture for electricity generation.

Food Waste

Food loss and waste is food that is not eaten. The causes of food waste or loss are numerous and occur throughout the food system, during production, processing, distribution, retail and food service sales, and consumption. Overall, about one-third of the world’s food is thrown away. Metaanalysis that did not include food lost during production, by the United Nations Environment Programme found that food waste was a challenge in all countries at all levels of economic development. The analysis estimated that global food waste was 931 million tonnes of food waste (about 121 kg per capita) across three sectors: 61 per cent from households, 26 per cent from food service and 13 per cent from retail.

Food loss and waste is a major part of the impact of agriculture on climate change (it amounts to 3.3 billion tons of CO2 e emissions annually) and other environmental issues, such as land use, water use and loss of biodiversity. Prevention of food waste is the highest priority, and when prevention is not possible, the food waste hierachy ranks the food waste treatment options from preferred to least preferred based on their negative environmental impacts. Reuse pathways of surplus food intended for human consumption, such as food donation, is the next best strategy after prevention, followed by animal feed, recycling of nutrients and energy followed by the least preferred option, landfill, which is a major source of the greenhouse gas methane. Other considerations include unreclaimed phosphorus in food waste leading to further phosphate mining. Moreover, reducing food waste in all parts of the food system is an important part of reducing the environmental impact of agriculture, by reducing the total amount of water, land, and other resources used.

 

Sewage Treatment

Primary treatment

Primary treatment removes material that will either float or readily settle out by gravity. It includes the physical processes of screening, comminution, grit removal, and sedimentation. Screens are made of long, closely spaced, narrow metal bars. They block floating debris such as wood, rags, and other bulky objects that could clog pipes or pumps. In modern plants the screens are cleaned mechanically, and the material is promptly disposed of by burial on the plant grounds. A comminutor may be used to grind and shred debris that passes through the screens. The shredded material is removed later by sedimentation or flotation processes.

Grit chambers are long narrow tanks that are designed to slow down the flow so that solids such as sand, coffee grounds, and eggshells will settle out of the water. Grit causes excessive wear and tear on pumps and other plant equipment. Its removal is particularly important in cities with combined sewer systems, which carry a good deal of silt, sand, and gravel that wash off streets or land during a storm.

Suspended solids that pass through screens and grit chambers are removed from the sewage in sedimentation tanks. These tanks, also called primary clarifiers, provide about two hours of detention time for gravity settling to take place. As the sewage flows through them slowly, the solids gradually sink to the bottom. The settled solids—known as raw or primary sludge are moved along the tank bottom by mechanical scrapers. Sludge is collected in a hopper, where it is pumped out for removal. Mechanical surface-skimming devices remove grease and other floating materials.

Secondary treatment

Secondary treatment removes the soluble organic matter that escapes primary treatment. It also removes more of the suspended solids. Removal is usually accomplished by biological processes in which microbes consume the organic impurities as food, converting them into carbon dioxide, water, and energy for their own growth and reproduction. The sewage treatment plant provides a suitable environment, albeit of steel and concrete, for this natural biological process. Removal of soluble organic matter at the treatment plant helps to protect the dissolved oxygen balance of a receiving stream, river, or lake.

There are three basic biological treatment methods: the trickling filter, the activated sludge process, and the oxidation pond. A fourth, less common method is the rotating biological contacter.

Trickling filter

A trickling filter is simply a tank filled with a deep bed of stones. Settled sewage is sprayed continuously over the top of the stones and trickles to the bottom, where it is collected for further treatment. As the wastewater trickles down, bacteria gather and multiply on the stones. The steady flow of sewage over these growths allows the microbes to absorb the dissolved organics, thus lowering the biochemical oxygen demand (BOD) of the sewage. Air circulating upward through the spaces among the stones provides sufficient oxygen for the metabolic processes.

Settling tanks, called secondary clarifiers, follow the trickling filters. These clarifiers remove microbes that are washed off the rocks by the flow of wastewater. Two or more trickling filters may be connected in series, and sewage can be recirculated in order to increase treatment efficiencies.

Activated sludge

The activated sludge treatment system consists of an aeration tank followed by a secondary clarifier. Settled sewage, mixed with fresh sludge that is recirculated from the secondary clarifier, is introduced into the aeration tank. Compressed air is then injected into the mixture through porous diffusers located at the bottom of the tank. As it bubbles to the surface, the diffused air provides oxygen and a rapid mixing action. Air can also be added by the churning action of mechanical propeller-like mixers located at the tank surface.

Under such oxygenated conditions, microorganisms thrive, forming an active, healthy suspension of biological solids—mostly bacteria—called activated sludge. About six hours of detention is provided in the aeration tank. This gives the microbes enough time to absorb dissolved organics from the sewage, reducing the BOD. The mixture then flows from the aeration tank into the secondary clarifier, where activated sludge settles out by gravity. Clear water is skimmed from the surface of the clarifier, disinfected, and discharged as secondary effluent. The sludge is pumped out from a hopper at the bottom of the tank. About 30 percent of the sludge is recirculated back into the aeration tank, where it is mixed with the primary effluent. This recirculation is a key feature of the activated sludge process. The recycled microbes are well acclimated to the sewage environment and readily metabolize the organic materials in the primary effluent. The remaining 70 percent of the secondary sludge must be treated and disposed of in an acceptable manner (see Sludge treatment and disposal).

Variations of the activated sludge process include extended aeration, contact stabilization, and high-purity oxygen aeration. Extended aeration and contact stabilization systems omit the primary settling step. They are efficient for treating small sewage flows from motels, schools, and other relatively isolated wastewater sources. Both of these treatments are usually provided in prefabricated steel tanks called package plants. Oxygen aeration systems mix pure oxygen with activated sludge. A richer concentration of oxygen allows the aeration time to be shortened from six to two hours, reducing the required tank volume.

Oxidation pond

Oxidation ponds, also called lagoons or stabilization ponds, are large, shallow ponds designed to treat wastewater through the interaction of sunlight, bacteria, and algae. Algae grow using energy from the sun and carbon dioxide and inorganic compounds released by bacteria in water. During the process of photosynthesis, the algae release oxygen needed by aerobic bacteria. Mechanical aerators are sometimes installed to supply yet more oxygen, thereby reducing the required size of the pond. Sludge deposits in the pond must eventually be removed by dredging. Algae remaining in the pond effluent can be removed by filtration or by a combination of chemical treatment and settling.

Effluent polishing

For the removal of additional suspended solids and BOD from secondary effluent, effluent polishing is an effective treatment. It is most often accomplished using granular media filters, much like the filters used to purify drinking water. Polishing filters are usually built as prefabricated units, with tanks placed directly above the filters for storing backwash water. Effluent polishing of wastewater may also be achieved using microstrainers of the type used in treating municipal water supplies.

 

Sludge Treatment and Disposal

The residue that accumulates in sewage treatment plants is called sludge (or biosolids). Sewage sludge is the solid, semisolid, or slurry residual material that is produced as a by-product of wastewater treatment processes. This residue is commonly classified as primary and secondary sludge. Primary sludge is generated from chemical precipitation, sedimentation, and other primary processes, whereas secondary sludge is the activated waste biomass resulting from biological treatments. Some sewage plants also receive septage or septic tank solids from household on-site wastewater treatment systems. Quite often the sludges are combined together for further treatment and disposal.

Treatment and disposal of sewage sludge are major factors in the design and operation of all wastewater treatment plants. Two basic goals of treating sludge before final disposal are to reduce its volume and to stabilize the organic materials. Stabilized sludge does not have an offensive odour and can be handled without causing a nuisance or health hazard. Smaller sludge volume reduces the costs of pumping and storage.

Treatment methods

Treatment of sewage sludge may include a combination of thickening, digestion, and dewatering processes.

Thickening

Thickening is usually the first step in sludge treatment because it is impractical to handle thin sludge, a slurry of solids suspended in water. Thickening is usually accomplished in a tank called a gravity thickener. A thickener can reduce the total volume of sludge to less than half the original volume. An alternative to gravity thickening is dissolved-air flotation. In this method, air bubbles carry the solids to the surface, where a layer of thickened sludge forms.

Digestion

Sludge digestion is a biological process in which organic solids are decomposed into stable substances. Digestion reduces the total mass of solids, destroys pathogens, and makes it easier to dewater or dry the sludge. Digested sludge is inoffensive, having the appearance and characteristics of a rich potting soil.

Most large sewage treatment plants use a two-stage digestion system in which organics are metabolized by bacteria anaerobically (in the absence of oxygen). In the first stage, the sludge, thickened to a dry solids (DS) content of about 5 percent, is heated and mixed in a closed tank for several days. Acid-forming bacteria hydrolyze large molecules such as proteins and lipids, breaking them into smaller water-soluble molecules, and then ferment those smaller molecules into various fatty acids. The sludge then flows into a second tank, where the dissolved matter is converted by other bacteria into biogas, a mixture of carbon dioxide and methane. Methane is combustible and is used as a fuel to heat the first digestion tank as well as to generate electricity for the plant.

Anaerobic digestion is very sensitive to temperature, acidity, and other factors. It requires careful monitoring and control. In some cases, the sludge is inoculated with extra hydrolytic enzymes at the beginning of the first digestion stage in order to supplement the action of the bacteria. It has been found that this enzymatic treatment can destroy more unwanted pathogens in the sludge and also can result in the generation of more biogas in the second stage of digestion.

Another enhancement of the traditional two-stage anaerobic digestion process is thermal hydrolysis, or the breaking down of the large molecules by heat. This is done in a separate step before digestion. In a typical case, the process begins with a sludge that has been dewatered to a DS content of some 15 percent. The sludge is mixed with steam in a pulper, and this hot homogenized mixture is fed to a reactor, where it is held under pressure at approximately 165 °C (about 330 °F) for about 30 minutes. At that point, with the hydrolytic reactions complete, some of the steam is bled off (to be fed to the pulper), and the sludge, still under some pressure, is released suddenly into a “flash tank,” where the sudded drop in pressure bursts the cell walls of much of the solid matter. The hydrolyzed sludge is cooled, diluted slightly with water, and then sent directly to the second stage of anaerobic digestion.

Sludge digestion may also take place aerobically—that is, in the presence of oxygen. The sludge is vigorously aerated in an open tank for about 20 days. Methane gas is not formed in this process. Although aerobic systems are easier to operate than anaerobic systems, they usually cost more to operate because of the power needed for aeration. Aerobic digestion is often combined with small extended aeration or contact stabilization systems.

Aerobic and conventional anaerobic digestion convert about half of the organic sludge solids to liquids and gases. Thermal hydrolysis followed by anaerobic digestion can convert some 60 to 70 percent of the solid matter to liquids and gases. Not only is the volume of solids produced smaller than in conventional digestion, but the greater production of biogas can make some wastewater treatment plants self-sufficient in energy.

Dewatering

Digested sewage sludge is usually dewatered before disposal. Dewatered sludge still contains a significant amount of water—often as much as 70 percent—but, even with that moisture content, sludge no longer behaves as a liquid and can be handled as a solid material. Sludge-drying beds provide the simplest method of dewatering. A digested sludge slurry is spread on an open bed of sand and allowed to remain until dry. Drying takes place by a combination of evaporation and gravity drainage through the sand. A piping network built under the sand collects the water, which is pumped back to the head of the plant. After about six weeks of drying, the sludge cake, as it is called, may have a solids content of about 40 percent. It can then be removed from the sand with a pitchfork or a front-end loader. In order to reduce drying time in wet or cold weather, a glass enclosure may be built over the sand beds. Since a good deal of land area is needed for drying beds, this method of dewatering is commonly used in rural or suburban towns rather than in densely populated cities.

Alternatives to sludge-drying beds include the rotary drum vacuum filter, the centrifuge, and the belt filter press. These mechanical systems require less space than do sludge-drying beds, and they offer a greater degree of operational control. However, they usually have to be preceded by a step called sludge conditioning, in which chemicals are added to the liquid sludge to coagulate solids and improve drainability.

Disposal

The final destination of treated sewage sludge usually is the land. Dewatered sludge can be buried underground in a sanitary landfill. It also may be spread on agricultural land in order to make use of its value as a soil conditioner and fertilizer. Since sludge may contain toxic industrial chemicals, it is not spread on land where crops are grown for human consumption.

Where a suitable site for land disposal is not available, as in urban areas, sludge may be incinerated. Incineration completely evaporates the moisture and converts the organic solids into inert ash. The ash must be disposed of, but the reduced volume makes disposal more economical. Air pollution control is a very important consideration when sewage sludge is incinerated. Appropriate air-cleaning devices such as scrubbers and filters must be used.

Dumping sludge in the ocean, once an economical disposal method for many coastal communities, is no longer considered a viable option. It is now prohibited in the United States and many other coastal countries.

 

Sewerage System

A sewerage system, or wastewater collection system, is a network of pipes, pumping stations, and appurtenances that convey sewage from its points of origin to a point of treatment and disposal.

Combined systems

Systems that carry a mixture of both domestic sewage and storm sewage are called combined sewers. Combined sewers typically consist of large-diameter pipes or tunnels, because of the large volumes of storm water that must be carried during wet-weather periods. They are very common in older cities but are no longer designed and built as part of new sewerage facilities. Because wastewater treatment plants cannot handle large volumes of storm water, sewage must bypass the treatment plants during wet weather and be discharged directly into the receiving water. These combined sewer overflows, containing untreated domestic sewage, cause recurring water pollution problems and are very troublesome sources of pollution.

In some large cities the combined sewer overflow problem has been reduced by diverting the first flush of combined sewage into a large basin or underground tunnel. After temporary storage, it can be treated by settling and disinfection before being discharged into a receiving body of water, or it can be treated in a nearby wastewater treatment plant at a rate that will not overload the facility. Another method for controlling combined sewage involves the use of swirl concentrators. These direct sewage through cylindrically shaped devices that create a vortex, or whirlpool, effect. The vortex helps concentrate impurities in a much smaller volume of water for treatment.

Separate systems

New wastewater collection facilities are designed as separate systems, carrying either domestic sewage or storm sewage but not both. Storm sewers usually carry surface runoff to a point of disposal in a stream or river. Small detention basins may be built as part of the system, storing storm water temporarily and reducing the magnitude of the peak flow rate. Sanitary sewers, on the other hand, carry domestic wastewater to a sewage treatment plant. Pretreated industrial wastewater may be allowed into municipal sanitary sewerage systems, but storm water is excluded.

Storm sewers are usually built with sections of reinforced concrete pipe. Corrugated metal pipes may be used in some cases. Storm water inlets or catch basins are located at suitable intervals in a street right-of-way or in easements across private property. The pipelines are usually located to allow downhill gravity flow to a nearby stream or to a detention basin. Storm water pumping stations are avoided, if possible, because of the very large pump capacities that would be needed to handle the intermittent flows.

A sanitary sewerage system includes laterals, submains, and interceptors. Except for individual house connections, laterals are the smallest sewers in the network. They usually are not less than 200 mm (8 inches) in diameter and carry sewage by gravity into larger submains, or collector sewers. The collector sewers tie in to a main interceptor, or trunk line, which carries the sewage to a treatment plant. Interceptors are usually built with precast sections of reinforced concrete pipe, up to 5 metres (15 feet) in diameter. Other materials used for sanitary sewers include vitrified clay, asbestos cement, plastic, steel, or ductile iron. The use of plastic for laterals is increasing because of its lightness and ease of installation. Iron and steel pipes are used for force mains or in pumping stations. Force mains are pipelines that carry sewage under pressure when it must be pumped.

Alternative Systems

Sometimes the cost of conventional gravity sewers can be prohibitively high because of low population densities or site conditions such as a high water table or bedrock. Three alternative wastewater collection systems that may be used under these circumstances include small-diameter gravity sewers, pressure sewers, and vacuum sewers.

In small-diameter gravity systems, septic tanks are first used to remove settleable and floating solids from the wastewater from each house before it flows into a network of collector mains (typically 100 mm, or 4 inches, in diameter); these systems are most suitable for small rural communities. Because they do not carry grease, grit and sewage solids, the pipes can be of smaller diameter and placed at reduced slopes or gradients to minimize trench excavation costs. Pressure sewers are best used in flat areas or where expensive rock excavation would be required. Grinder pumps discharge wastewater from each home into the main pressure sewer, which can follow the slope of the ground. In a vacuum sewerage system, sewage from one or more buildings flows by gravity into a sump or tank from which it is pulled out by vacuum pumps located at a central vacuum station and then flows into a collection tank. From the vacuum collection tank the sewage is pumped to a treatment plant.

Pumps

Pumping stations are built when sewage must be raised from a low point to a point of higher elevation or where the topography prevents downhill gravity flow. Special nonclogging pumps are available to handle raw sewage. They are installed in structures called lift stations. There are two basic types of lift stations: dry well and wet well. A wet-well installation has only one chamber or tank to receive and hold the sewage until it is pumped out. Specially designed submersible pumps and motors can be located at the bottom of the chamber, completely below the water level. Dry-well installations have two separate chambers, one to receive the wastewater and one to enclose and protect the pumps and controls. The protective dry chamber allows easy access for inspection and maintenance. All sewage lift stations, whether of the wet-well or dry-well type, should include at least two pumps. One pump can operate while the other is removed for repair.

Flow rates

There is a wide variation in sewage flow rates over the course of a day. A sewerage system must accommodate this variation. In most cities domestic sewage flow rates are highest in the morning and evening hours. They are lowest during the middle of the night. Flow quantities depend upon population density, water consumption, and the extent of commercial or industrial activity in the community. The average sewage flow rate is usually about the same as the average water use in the community. In a lateral sewer, short-term peak flow rates can be roughly four times the average flow rate. In a trunk sewer, peak flow rates may be two-and-a-half times the average.

Although sewage flows depend upon residential, commercial, and industrial connections, sewage flow rates potentially can become higher as a result of inflows and infiltration (I&I) into the sanitary sewer system. Inflows correspond to storm water entering sewers from inappropriate connections, such as roof drains, storm drains, downspouts and sump pumps. High amounts of rainwater runoff can reach the sewer system during precipitation and stormflow events or during seasonal spring flooding of rivers inundated with melting ice. Infiltration refers to the groundwater entering sewers via defective or broken pipes. In both these cases, downstream utilities and treatment plants may experience flows higher than anticipated and can become hydraulically overloaded. During such overloads, utilities may ask residents connected to the system to refrain from using dishwashers and washing machines and may even limit toilet flushing and the use of showers in an attempt to lessen the strain. Such I&I issues can be especially severe in old and aging water infrastructures.

 

Sewage Characteristics

Types of sewage

There are three types of wastewater, or sewage: domestic sewage, industrial sewage, and storm sewage. Domestic sewage carries used water from houses and apartments; it is also called sanitary sewage. Industrial sewage is used water from manufacturing or chemical processes. Storm sewage, or storm water, is runoff from precipitation that is collected in a system of pipes or open channels.

Domestic sewage is slightly more than 99.9 percent water by weight. The rest, less than 0.1 percent, contains a wide variety of dissolved and suspended impurities. Although amounting to a very small fraction of the sewage by weight, the nature of these impurities and the large volumes of sewage in which they are carried make disposal of domestic wastewater a significant technical problem. The principal impurities are putrescible organic materials and plant nutrients, but domestic sewage is also very likely to contain disease-causing microbes. Industrial wastewater usually contains specific and readily identifiable chemical compounds, depending on the nature of the industrial process. Storm sewage carries organic materials, suspended and dissolved solids, and other substances picked up as it travels over the ground.

Principal Pollutants

Organic material

The amount of putrescible organic material in sewage is indicated by the biochemical oxygen demand, or BOD; the more organic material there is in the sewage, the higher the BOD, which is the amount of oxygen required by microorganisms to decompose the organic substances in sewage. It is among the most important parameters for the design and operation of sewage treatment plants. Industrial sewage may have BOD levels many times that of domestic sewage. The BOD of storm sewage is of particular concern when it is mixed with domestic sewage in combined sewerage systems.

Dissolved oxygen is an important water quality factor for lakes and rivers. The higher the concentration of dissolved oxygen, the better the water quality. When sewage enters a lake or stream, decomposition of the organic materials begins. Oxygen is consumed as microorganisms use it in their metabolism. This can quickly deplete the available oxygen in the water. When the dissolved oxygen levels drop too low, trout and other aquatic species soon perish. In fact, if the oxygen level drops to zero, the water will become septic. Decomposition of organic compounds without oxygen causes the undesirable odours usually associated with septic or putrid conditions.

Suspended solids

Another important characteristic of sewage is suspended solids. The volume of sludge produced in a treatment plant is directly related to the total suspended solids present in the sewage. Industrial and storm sewage may contain higher concentrations of suspended solids than domestic sewage. The extent to which a treatment plant removes suspended solids, as well as BOD, determines the efficiency of the treatment process.

Plant nutrients

Domestic sewage contains compounds of nitrogen and phosphorus, two elements that are basic nutrients essential for the growth of plants. In lakes, excessive amounts of nitrates and phosphates can cause the rapid growth of algae. Algal blooms, often caused by sewage discharges, accelerate the natural aging of lakes in a process called eutrophication.

Microbes

Domestic sewage contains many millions of microorganisms per gallon. Most are coliform bacteria from the human intestinal tract, and domestic sewage is also likely to carry other microbes. Coliforms are used as indicators of sewage pollution. A high coliform count usually indicates recent sewage pollution.

 

Rotating Biologycal Contactor (RBC)

RBC or Rotating Biological Contactor is a liquid waste treatment process using a method in which this wastewater treatment unit rotates with a center on an axis or axle which is driven by a motor drive system and/or air blowing (air drive system) from a diffuser immersed in wastewater. , under media. Made of plastic, the media for attachment of microbes is installed in such a way that there is the widest possible contact with waste water and oxygen that occurs alternately. Where the method involves contact with biological elements in rotation or rotation.

RBC is like a collection of discs where on the surface there is a disk media as a place for microorganisms to eat the organic matter content in the waste, it is cultivated that the disk media can be provided as widely as possible so that microorganisms can easily take pollutants in the flowing waste. The RBC operating system uses microorganisms to eat organic matter. Microorganisms need food and O2 to survive. So that the RBC is set like a spinning wheel so that when it is below the microorganism can take food while it can process it by taking oxygen first when it is above. However, it should also be noted that if RBC has been used for a long time on the surface of the disk media, large piles of microorganisms will form due to the growth of MO (microorganisms). if this MO continues to pile up, then the MO in the lowest pile that lives is only an aerobic MO because it is covered by the MO above it. so sometimes it forms like a crust.

RBC Working Principles

The working principle of wastewater treatment with RBC is that wastewater containing organic pollutants is contacted with a layer of micro-organisms (microbial film) attached to the surface of the media in a reactor. The media where the biological film is attached is in the form of a disk (disk) made of lightweight polymer or plastic and arranged in a row on an axis to form a module or package, then the module is rotated slowly in a state partially immersed in flowing wastewater. continuously into the reactor.

In this way micro-organisms such as bacteria, algae, protozoa, fungi, and others grow attached to the surface of the rotating medium forming a layer consisting of micro-organisms called biofilm (biological layer). Micro-organisms will decompose or take organic compounds in water and take oxygen dissolved in water or from the air for their metabolic processes, so that the content of organic compounds in wastewater is reduced.

When the biofilm attached to the medium in the form of a thin disc is immersed in wastewater, micro-organisms absorb organic compounds present in the wastewater flowing on the surface of the biofilm, and when the biofilm is above the water surface, the micro-organisms absorb oxygen from the surface of the biofilm. air or oxygen dissolved in water to decompose organic compounds. The energy resulting from the decomposition of organic compounds is used by micro-organisms for the process of reproduction or metabolism.

Compounds resulting from the metabolic processes of these micro-organisms will come out of the biofilm and be carried away by the flow of water or in the form of gas and will be dispersed into the air through the cavities in the medium, while suspended solids (SS) will be retained on the surface of the biological layer (biofilm). ) and will decompose into water-soluble forms.

The growth of these micro-organisms or biofilms is getting thicker and thicker, until finally due to gravity some of it will peel off from the medium and be carried out by the water flow. Furthermore, micro-organisms on the surface of the medium will grow again by itself until an equilibrium occurs according to the content of organic compounds present in the wastewater.

Aplication of Rotating Biological Contactor (RBC)

The performance of the RBC depends also on the number of compartments. One module can contain four or five compartments. In the first compartment, backflow can be added to the initial settling unit so that the conditions are not too anaerobic so that the bad smell is reduced while helping the dynamics of microbial growth. Likewise, in the final compartment a backflow to the initial settling unit may be installed for the same purpose. Generally, the RBC contact medium is submerged in wastewater as high as 40% of its diameter. The rotation speed is between 1 – 3 revolutions per minute. This rotation provides sufficient energy for the hydraulic force to dislodge the biofilm and the water flow is turbulent so that the solid remains suspended (does not settle). The hydraulic residence time in each module is relatively short, ie 20 minutes at normal load. Each stage or module tends to operate as a completely stirred reactor.

Regarding the microbial adhesive media, there are several materials that can be used. What is often chosen is HDPE (high-density polyethylene) plastic media with a diameter of 2-4 m, with a thickness of up to 10 mm. The form of media can be in the form of plates but can also be in the form of pipes or tubes mounted on an iron shaft with a span of up to 8 m. The media along with the shaft and the motor is called a module that keeps on rotating in the tub. Several modules can be installed in series or parallel according to the discharge requirements of the treated wastewater. Usually the modules are separated by a baffle to avoid short circuiting in the tank. RBC performance is also influenced by wastewater temperature, influent substrate concentration, hydraulic residence time, ratio of tank volume to media surface area, media rotation speed, and dissolved oxygen.

Generally, to treat RBC domestic wastewater, it does not require microbial seeding. This is because the microbes are already available in sufficient quantities to start the process. Approximately a week to two weeks after starting the processing, on the surface of the media will stick biomass with a thickness of 1-4 mm. This thickness depends on the strength of the wastewater and the rotational speed of the adhesive medium. According to Antonie, 1978, the concentration of these microbes reached 50,000 – 100,000 mg/l, a very high amount so that quite a lot of organic pollutants and nitrogen were removed with the help of dissolved oxygen.

Waste Treatment Process with RBC

In general, the wastewater treatment process with the RBC system consists of a sand separator, initial settling basin, flow control tank, rotary biological reactor/contactor (RBC), final settling basin, chlorination tank, and a sludge treatment unit.

Sand Separator Tub. Wastewater flows quietly into a sand separator, so that dirt in the form of sand or coarse silt can be deposited. Meanwhile, floating dirt such as garbage, plastic, cloth waste and others are stuck in the screen installed at the inlet of the sand separator pool.

Early Sediment Tub. From the separator/sand settler tank, wastewater is drained to the initial sealing tank. In this initial settling tank mud or suspended solids mostly settle. The residence time in the initial sealing tank is 2 – 4 hours, and the sediment that has settled is collected and pumped into the sludge deposition tank.

Flow Control Body. If the wastewater flow rate exceeds the planning capacity, the excess wastewater discharge is channeled to a flow control tank for temporary storage. When the flow rate is low, the wastewater in the control tank is pumped into the initial settling basin together with the new wastewater according to the desired discharge.

Biological contactor (reactor) Swivel. In this contactor bath, the medium is a thin disk (disk) of polymer or plastic material in large quantities, which is attached or assembled on a shaft, rotated slowly in a state partially immersed in the wastewater. The residence time in the contactor bath is approximately 2.5 hours. Under such conditions, micro-organisms will grow on the surface of the rotating medium, forming a biological film. The biological film consists of various types/spicies of micro-organisms such as bacteria, protozoa, fungi, and others. Micro-organisms that grow on the surface of this media will decompose organic compounds in the wastewater. The biological layer is getting thicker and thicker because of the gravity it will peel off by itself and the organic mud will be carried away by the water flow out. Furthermore, the biological layer will grow and develop again on the surface of the media by itself.

Final Precipitation Tub. The wastewater that comes out of the contactor (reactor) tank is then channeled to the final settling basin, with a settling time of about 3 hours. Compared to the activated sludge process, the sludge from RBC is easier to settle, because it is larger in size and heavier. The runoff water (over flow) from the final settling basin is relatively clear, then it is flowed into the chlorination tank. Meanwhile, the sludge that settles at the bottom of the tank is pumped to the sludge concentration tank together with the mud from the initial settling basin.

Chlorination tub. Treated water or runoff water from the final settling basin still contains coli bacteria, pathogenic bacteria, or viruses that have the potential to infect the surrounding community. To overcome this, the wastewater that comes out of the final settling basin is channeled into a chlorination tank to kill pathogenic micro-organisms present in the water. In the chlorination tank, the wastewater is spiked with chlorine compounds with a certain dose and contact time so that all pathogenic micro-organisms can be killed. Furthermore, from the chlorination tank, wastewater can be discharged into water bodies.

Mud Concentration Tub. Sludge from the initial settling basin and the final settling basin is collected in a sludge concentration basin. In the tank, the mud is stirred slowly and then concentrated by allowing it to stand for about 25 hours so that the mud settles, then the supernatant water at the top is flowed into the initial settling basin, while the concentrated mud is pumped into the mud dryer or accommodated in the tank. separately and periodically sent to a sludge treatment center elsewhere.

Reaction on RBC

In the RBC process, there are several reactions that occur, namely:
1. Oxidation
2. Nitrification
3. Denitrification

This is described as follows. Organic material contained in the waste then takes oxygen so that there is a reaction between organic matter, O2 and nutrients (usually already contained in the waste) in the metabolic process and then NH3, CO2, C5H7HO2 (new cells) are released into the air.

In addition to the above process, there is endogenous respiration to obtain energy, namely:

C5H7HO2 + O2   –>>    5CO2   +   H2O   +   Energy

in the nitrification of waste has a pollutant containing ammonia NH4 which smells very pungent. with the following reaction:

2NH4   +   O2 (with the help of nitrosomonas)  —>>   2NO2 + 4H + 2H2O