Chemical treatment of wastewater

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“Pollution is nothing but the resources we are not harvesting. We allow them to disperse because we’ve been ignorant of their value.” – R. Buckminster Fuller

Chemicals are used during wastewater treatment in an array of processes to expedite disinfection. These chemical processes, which induce chemical reactions, are called chemical unit processes, and are used alongside biological and physical cleaning processes to achieve various water standards. There are several distinct chemical unit processes, including chemical coagulation, chemical precipitation, chemical oxidation and advanced oxidation, ion exchange, and chemical neutralization and stabilization, which can be applied to wastewater during cleaning.

Wastewater Chemical Treatment

Chemical Precipitation

Chemical precipitation is the most common method for removing dissolved metals from wastewater solution containing toxic metals. To convert the dissolved metals into solid particle form, a precipitation reagent is added to the mixture. A chemical reaction, triggered by the reagent, causes the dissolved metals to form solid particles. Filtration can then be used to remove the particles from the mixture. How well the process works is dependent upon the kind of metal present, the concentration of the metal, and the kind of reagent used. In hydroxide precipitation, a commonly used chemical precipitation process, calcium or sodium hydroxide is used as the reagent to create solid metal hydroxides. However, it can be difficult to create hydroxides from dissolved metal particles in wastewater because many wastewater solutions contain mixed metals.

Chemical Coagulation

This chemical process involves destabilizing wastewater particles so that they aggregate during chemical flocculation. Fine solid particles dispersed in wastewater carry negative electric surface charges (in their normal stable state), which prevent them from forming larger groups and settling. Chemical coagulation destabilizes these particles by introducing positively charged coagulants that then reduce the negative particles’ charge. Once the charge is reduced, the particles freely form larger groups. Next, an anionic flocculant is introduced to the mixture. Because the flocculant reacts against the positively charged mixture, it either neutralizes the particle groups or creates bridges between them to bind the particles into larger groups. After larger particle groups are formed, sedimentation can be used to remove the particles from the mixture.

e.g. Alum and ferric (or ferrous) sulphate are examples of inorganic coagulants.

The optimum pH for alum treatment is 6 to 7 while that for Iron(III)sulfate is 6 to11. Therefore if the pH of wastewater is say 8, we have to use the latter one or neutralize the wastewater to get the pH to 6 or 7 and then use alum, whatever works.

Chemical Oxidation and Advanced Oxidation

With the introduction of an oxidizing agent during chemical oxidation, electrons move from the oxidant to the pollutants in wastewater. The pollutants then undergo structural modification, becoming less destructive compounds. Alkaline chlorination uses chlorine as an oxidant against cyanide. However, alkaline chlorination as a chemical oxidation process can lead to the creation of toxic chlorinated compounds, and additional steps may be required. Advanced oxidation can help remove any organic compounds that are produced as a byproduct of chemical oxidation, through processes such as steam stripping, air stripping, or activated carbon adsorption.

Ion Exchange

When water is too hard, it is difficult to use to clean and often leaves a grey residue. (This is why clothing washed in hard water often retains a dingy tint.)  An  ion exchange process can be used to soften the water. Calcium and magnesium are common ions that lead to water hardness. To soften the water, positively charged sodium ions are introduced in the form of dissolved sodium chloride salt, or brine. Hard calcium and magnesium ions exchange places with sodium ions, and free sodium ions are simply released in the water. However, after softening a large amount of water, the softening solution may fill with excess calcium and magnesium ions, requiring the solution be recharged with sodium ions.

Chemical Stabilization

This process works in a similar fashion as chemical oxidation. Sludge is treated with a large amount of a given oxidant, such as chlorine. The introduction of the oxidant slows down the rate of biological growth within the sludge, and also helps deodorize the mixture. The water is then removed from the sludge. Hydrogen peroxide can also be used as an oxidant, and may be a more cost-effective choice.

Reference: Thomasnet

Biological treatment of wastewater

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“Don’t forget that the flavors of wine and cheese depend upon the types of infecting microorganisms.” – Martin H. Fischer

Biological treatment aka secondary treatment:

In wastewater treatment, the treatment process that follows primary treatment. It is used to remove the remaining organic solids that have not been removed in primary treatment together with the 90% or more of the dissolved organics. Aerobic biological treatment is commonly used. Secondary treatment may also incorporate nitrification and biological phosphorus removal.

Reference: Dictionary of waste and water management, Elsevier

Secondary treatment, usually biological, tries to remove the remaining dissolved or colloidal organic matter. Generally, the biodegradation of the pollutants is allowed to take place in a location where plenty of air can be supplied to the microorganisms. This promotes formation of the less offensive, oxidized products. Engineers try to design the capacity of the treatment units so that enough of the impurities will be removed to prevent significant oxygen demand in the receiving water after discharge.

There are two major types of biological treatment processes:

  1. attached growth
  2. suspended growth

In an attached growth process, the microorganisms grow on a surface, such as rock or plastic. Examples include open trickling filters, where the water is distributed over rocks and trickles down to underdrains, with air being supplied through vent pipes; enclosed biotowers, which are similar, but more likely to use shaped, plastic media instead of rocks; and so-called rotating biological contacters, or RBC’s, which consist of large, partially submerged discs which rotate continuously, so that the microorganisms growing on the disc’s surface are repeatedly being exposed alternately to the wastewater and to the air. The most common type of suspended growth process is the so-called activated sludge system. This type of system consists of two parts, an aeration tank and a settling tank, or clarifier. The aeration tank contains a “sludge” which is what could be best described as a “mixed microbial culture”, containing mostly bacteria, as well as protozoa, fungi, algae, etc. This sludge is constantly mixed and aerated either by compressed air bubblers located along the bottom, or by mechanical aerators on the surface. The wastewater to be treated enters the tank and mixes with the culture, which uses the organic compounds for growth– producing more microorganisms– and for respiration, which results mostly in the formation of carbon dioxide and water. The process can also be set up to provide biological removal of the nutrients nitrogen and phosphorus. Refer to Figure 1 for a simplified process flow sheet.

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Reference: Handbook of Waste water treatment technologies by Nicholas Cherimisinoff

Some people are rather skeptical when it comes to biological treatment. Here’s why:

  • Slow rate of treatment
  • Large volumes require more floor area
  • Often needs engineered microorganisms which translates into shelling out a lot of money

Read more:

Ways of treating waste

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Volunteer Image Author Jemzo

One can treat waste in three ways in order of preference:

  1. Mechanically – Primary
  2. Biologically – Secondary
  3. Chemically – Tertiary

From a broad perspective, these three ways can be further divided into the following ways:

Physical:

  • Sedimentation (Clarification)
  • Screening
  • Aeration
  • Filtration
  • Flotation and Skimming
  • Degassification
  • Equalization

Chemical:

  • Chlorination
  • Ozonation
  • Neutralization
  • Coagulation
  • Adsorption
  • Ion Exchange

Biological:

Aerobic

  • Activated Sludge Treatment Methods
  • Trickling Filtration
  • Oxidation Ponds
  • Lagoons
  • Aerobic Digestion

Anaerobic

  • Anaerobic Digestion
  • Septic Tanks
  • Lagoons

This post will look at the first one in detail followed by the two in the next articles.

So, how to remove mechanical debris from waste water?

This is how:

PRE-TREATMENT (SCREENING):

Wastewater is pre-treated to remove large floating and suspended solids which could interfere with the normal operation of subsequent treatment processes.

Pre-treatment consists of:

  1. Screening
  2. Grit removal

Screens of various sizes and shapes are used, depending on the nature of solids to be removed, and cleaning is done either manually or mechanically. Fixed bar screens are the most common type of screens used in domestic wastewater treatment facilities. Bar screens are made up of parallel metal bars and have apertures in the range 20-60 mm for coarse screens and 10-20 mm for fine and medium screens. Where a fine screen is employed, it is usually preceded by a coarse screen to remove large solids in order to avoid clogging problems. Methods of disposal include burial, incineration, grinding and digestion. To avoid disposal problems, some treatment plants use a device known as comminutor instead of the screens. The comminutor grinds large solids which can then be satisfactorily handled in the sedimentation tank. After screening, the wastewater enters a grit chamber for the removal of inorganic grit, consisting of sand, gravel, cinder and pebbles. Grit chambers are provided to protect pumps from abrasion and to reduce the formation of heavy deposits in pipes and channels. The grit can be removed by scrappers. When comminutors are used in place of bar screens, they are generally placed after the grit chambers. [1]

GRAVITATIONAL SETLLING:

Settleable solids are removed by gravitational settling under quiescent conditions. The sludge formed at the bottom of the tank is removed as underflow either by vacuum suction of by raking it to a discharge point at the bottom of the tank for withdrawal. The clear liquid produced is known as the overflow and it should contain no readily settleable matter.  [1] The process of sedimentation involves the separation from water, by gravitational settling of suspended particles that are heavier than water. The resulting effluent is then subject to rapid filtration to separate out solids that are still suspended in the water. Rapid filters typically consist of 24 to 36 inches of 0.5 to 1-mm diameter sand and/or anthracite. Particles are removed as water is filtered through the media at rates of 1 to 6 gallons/minute/square foot. Rapid filtration is effective in removing most particles that remain after sedimentation. The substances that are removed by coagulation, sedimentation, and filtration accumulate in sludge which must be properly disposed of. [2]

Types of sedimentation tanks used for this operation are:

  1. Rectangular horizontal flow
  2. Circular radial flow
  3. Vertical flow [1]

FLOATATION:

Floatation may be used in place of sedimentation, primarily for treating industrial wastewaters containing finely divided suspended solids and oily matter.

Floatation is used in:

  1. Paper Industry: To recover fine fibres from the screened effluent.
  2. Oil Industry: For clarification of oil-bearing waste.
  3. Treating effluents from tannery, metal finishing, cold-rolling, and pharmaceutical industries. [1]

FLOCCULATION:

The process of flocculation is applicable to aqueous waste streams where particles must be agglomerated into larger more settleable particles prior to sedimentation or other types of treatment. Highly viscous waste streams will inhibit the settling of solids. In addition to being used to treat waste streams, precipitation can also be used as an in situ process to treat aqueous wastes in surface impoundments. In an in-situ application, lime and flocculants are added directly to the lagoon, and mixing, flocculation, and sedimentation are allowed to occur within the lagoon. [2]

MICRO STRAINING:

Microstraining utilizes a rotating drum-type filter to screen suspended solids. The filtering media consist of a finely woven stainless steel fabric with a mesh size of 23 to 35 microns. The fabric is mounted on the periphery of the drum and water is allowed to pass from inside to the outside. Back-washing is accomplished by high pressure water jets placed at the highest point of the drum. The solids which are retained on the fabric are washed into a trough, which recycles the solids to the sedimentation tank. [2]

MICROFILTERS:

Microfilters are small-scale filters designed to remove cysts, suspended solids, protozoa, and, in some cases, bacteria from water. Most filters use a ceramic or fiber element that can be cleaned to restore performance as the units are used. [2]

SAND FILTERS:

A typical sand filter system consists of two or three chambers or basins. The first is the sedimentation chamber, which removes floatables and heavy sediments. The second is the filtration chamber, which removes additional pollutants by filtering the runoff through a sand bed. The third is the discharge chamber. The treated filtrate normally is then discharged through an underdrain system either to a storm drainage system or directly to surface waters. Sand filters are able to achieve high removal efficiencies for sediment, biochemical oxygen demand (BOD), and faecal coliform bacteria. Total metal removal, however, is moderate, and nutrient removal is often low. Figure below illustrates one type of configuration. Typically, sand filters begin to experience clogging problems within 3 to 5 years. Accumulated trash, paper, debris should be removed every six months or as needed. Corrective maintenance of the filtration chamber includes removal and replacement of the top layers of sand and gravel as they become clogged. [2]

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Figure: Example of a sand filter configuration [2]

REFERENCES:

  1. Environmental Pollution Control Engineering, by C.S.Rao
  2. Handbook of Water and Wastewater Treatment Technologies by Nicholas P. Cheremisinoff

Read more: Wastewater Treatment Methods and Disposal

Aerobic treatment of wastewater

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Volunteer Image Author Maark

INTRODUCTION:

Definition:

It is any treatment process that uses aerobes to biodegrade or oxidise or remove the unwanted organic or inorganic compounds.  In the treatment of wastewaters, the processes can be divided into suspended growth processes (e.g. activated sludge process, Figure 6) or attached growth processes such as rotating biological contactors (Figure 7) or trickling filters (Figure 8). [1]

Aerobe is a micro-organism that needs free or dissolved oxygen to develop. [1]

TREATMENT PROCESS:

Activated sludge process:

A continuous, aerobic biological treatment for wastewater dating from 1913, that uses a culture of bacteria suspended in the wastewater in an aeration tank to adsorb, absorb and biodegrade the organic pollutants. Flocs are formed that may reach 0.1 mm in diameter in the aeration tank. These are kept in suspension either by air blown into the bottom of the tank (diffused air system) or by mechanical aeration (see Figure 6). The mixture of the activated sludge and the wastewater in the aeration tank is known as the mixed liquor. The concentration of mixed liquor is known as mixed liquor suspended solids (MLSS), but is also measured as mixed liquor volatile suspended solids (MLVSS). The mixed liquor flows from the aeration tank to a sedimentation tank, where the activated sludge flocs combine together into larger particles that settle as a sludge. Most of the sludge from the sedimentation tank returns to the aeration tank. The dissolved oxygen in the aeration tank should be at least 0.5 mg/l, preferably 1 to 2 mg/l (see oxgen activated sludge). The aeration tank may be designed on aeration periodpreferably on F:M ratio or mean cell residence time. If the ammonia present is to be oxidised to nitrate, the plant must be designed for nitrification. In municipal wastewater, the activated sludge process is usually preceded by primary sedimentation. In the start up of an activated sludge plant, the time needed for establishing the appropriate bacteria and protozoa can be greatly reduced by seeding the new aeration tank with sludge from one that is working well. Activated sludge treatment demands only about one seventh of the land occupied by trickling filters. It also does not suffer from filter flies and has little smell. The operating cost of the aeration tank can be high and skilled attention is essential. Many varieties of activated sludge treatment exist, including contact stabilisation, extended aeration, modified aeration, oxygen activated sludge. [1]

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Figure 6: Activated Sludge process, flow diagram. [1]

Rotating Biological Contactor, RBC:

It is an aerobic wastewater treatment in which a horizontal shaft just above the surface of the wastewater carries closely spaced discs up to 3 m diameter or random plastics media in circular wire cages, that revolve with the shaft (see Figure 7). Some 25 to 45% of the discs or other media are submerged. The slow rotation develops a biofilm which oxidises the wastewater. When the slime layer becomes too thick, it sloughs off and is settled in a separate tank or compartment. In some plants, especially those for small communities, the discs are contained in the same tank as the zones for primary and secondary sedimentation and sludge storage. Some anaerobic sludge digestion will occur and the plant is desludged two to four times yearly. Little maintenance is needed.

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Figure 7: Rotating biological contactor [1]

Trickling Filter:

It is an aerobic biological treatment for wastewater that was developed in Salford, England in 1893. In a typical standard rate trickling filter (Figure 8), the wastewater trickles down through a 2 m deep bed of coarse stones, 24 to 100 mm diameter. Many other types exist, possibly using plastic media in a packed tower, as in high rate trickling filters. It is an attached growth process with a biofilm developing on the media. The attractions of the standard rate trickling filter are its low power cost and simplicity. Its disadvantages are the capital cost for large works, the large land area compared with activated sludge and the absence of versatility in operation. The trickling filter is not wholly aerobic. After the biofilm has formed, there is an anaerobic layer at the surface of the stones which little or no oxygen can reach. The main micro-organisms are aerobes, anaerobes and facultative anaerobic bacteria, but fungi are present also on aerobic surfaces, where they compete with bacteria for food. Algae occur at the top of the filter where there is sunlight. Trickling filters have been used to nitrify activated sludge effluent, and anaerobic filters can be used for denitrification. [1]

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Figure 8: Trickling filter [1]

ADVANTAGES:

The advantages of aerobic treatment relative to anaerobic treatment are as follows:

  1. Good process stability
  2. High effluent quality
  3. Smaller reactor sizes [4]

DISADVANTAGES:

The disadvantages of aerobic treatment relative to anaerobic treatment are as follows:

  1. High energy consumption
  2. Higher nutrient requirements
  3. High sludge yield
  4. High operating costs [4]

RELATIVE ADVANTAGES AND DISADVANTAGES OF ANAEROBIC AND AEROBIC PROCESSES

Process Anaerobic Aerobic 
Advantages
  • High energy consumption
  • Higher nutrient requirements
  • High sludge yield
  • High operating costs
  • Good process stability
  • High effluent quality
  • Smaller reactor sizes
Disadvantages
  • Low sludge yield
  • Low energy consumption
  • Generation of biogas
  • Low nutrient requirements
  • Sensitivity to toxicity and influent fluctuation
  • Requires more monitoring
  • Higher capital costs
  • Usually requires downstream aerobic polishing prior to discharge

Table 1: Relative advantages and disadvantages of anaerobic and aerobic processes [4]

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Figure 9: Fate of carbon and energy in Aerobic and Anaerobic wastewater treatment. [5]

REFERENCES:

  1. Dictionary of Water and Waste Management, Elsevier
  2. Environmental Engineers’ Handbook
  3. Wastewater treatment: biological and chemical processes By M. Henze
  4. Environmental Bioengineering By Lawrence K. Wang, Joo-Hwa Tay, Stephen Tiong Lee Tay
  5. Biological wastewater treatment: principles, modelling and design By M. Henze

Anaerobic treatment of wastewater

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Volunteer Image Author Curioso_Photography

INTRODUCTION:

Definition:

Any treatment process that uses anaerobes to remove unwanted organic or inorganic compounds is known as anaerobic treatment. [1] Anaerobe is a micro-organism that needs no free oxygen to develop. [1]

Description:

The first anaerobic treatment plant for industrial wastewater was built in 1929 for the treatment of wastewater from yeast production in Slagelse, Denmark. Even if this plant was in operation for almost 30 years, the development went very slowly. The development of this process did not gather momentum until the Dutch UASB plant type was introduced in 1980. [3]

Anaerobic microorganisms do not require DO in the water to function. They obtain their oxygen requirement from the oxygen chemically contained in organic materials. Anaerobic decomposition involves two separate but interrelated steps. First, the acid-producing bacteria decompose the dissolved organic waste to organic acids, such as acetic, propionic, and butyric acid. The organic acids are then further decomposed by methane producing bacteria to the end products of methane, carbon dioxide, and water. Effective operation requires a balance between acid production and breakdown because methane producers are sensitive to the concentration of volatile acids. [2]

APPLICATION:

Anaerobic treatment applies to both wastewater treatment and sludge digestion. [2]

Anaerobic treatment applied to wastewater treatment:

Anaerobic wastewater treatment is an effective biological method for treating many organic wastes. The microbiology involved in the process includes facultative and anaerobic microorganisms, which, in the absence of oxygen, convert organic materials into gaseous end products such as carbon dioxide and methane. Anaerobic wastewater treatment was discovered in the middle of the last century; however, environmental engineers have only seriously considered it in the last twenty years. Despite intense research in this field in the past few decades, much research is still needed in several areas. These areas include:

  1. Microbiology: Further research on the biochemistry and genetics related to the anaerobic microbial species is required.
  2. Startup procedures: Optimal procedures to minimize the lag time between the commissioning of a reactor and its placement into full operation must be investigated.
  3. Optimization of process engineering: Further optimization of the anaerobic treatment process is required, especially involving ancillary equipment, small-scale reactors, and support media (where applicable). [2]

PROCESS MICROBIOLOGY:

The end products of anaerobic degradation are gases, mostly methane (CH4), carbon dioxide (CO2), and small quantities of hydrogen sulfide (H2S) and hydrogen (H2).

The process involves two distinct stages:

  1. acid fermentation
  2. methane fermentation

In acid fermentation, the extracellular enzymes of a group of heterogenous and anaerobic bacteria hydrolyze complex organic waste components (proteins, lipids, and carbohydrates) to yield small soluble products. These simple, soluble compounds (e.g., triglycerides, fatty acids, amino acids, and sugars) are further subjected, by the bacteria, to fermentation, b-oxidations, and other metabolic processes that lead to the formation of simple organic compounds, mainly short-chain (volatile) acids (e.g., acetic [CH3COOH], propionic [CH3CH2COOH], butyric [CH3-CH2-CH2-COOH]) and alcohols. In the acid fermentation stage, no COD or BOD reduction is realized since this stage merely converts complex organic molecules to shortchain fatty acids, alcohols, and new bacterial cells, which exert an oxygen demand.

In the second stage, short-chain fatty acids (other than acetate) are converted to acetate, hydrogen gas, and carbon dioxide—a process referred to as acetogenesis. Subsequently, several species of strictly anaerobic bacteria bring about methanogenesis—a process in which hydrogen produces methane from acetate and carbon dioxide reduction. In this stage, the stabilization of the organic material truly occurs. Figure 1 shows the two stages of anaerobic treatment as sequential processes; however, both stages occur simultaneously and synchronously in an active, well-buffered system.

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Figure 1: Reaction pathways of anaerobic treatment of complex organic matter. [2]

The main concern of a wastewater treatment facility in operating an anaerobic system is that the various bacterial species function in a balanced and sequential way. Hence, although other types of microorganisms may be present in the reactors, attention is focused mostly on the bacteria.

The major groupings of bacteria, as numbered in Figure 1, and the reactions they mediate are as follows (Pavlostathis and Giraldo-Gomez 1991):

(1) fermentative bacteria,

(2) hydrogen-producing acetogenic bacteria,

(3) hydrogen-consuming acetogenic bacteria,

(4) carbon-dioxide-reducing methanogens, and

(5) aceticlastic methanogens.

Two common genera of aceticlastic methanogens are Methanothrix and Methanosarcina; and species from the Methanobacterium group are commonly known to produce methane by hydrogen reduction of carbon dioxide.

TREATMENT PROCESS:

The anaerobic wastewater treatment processes discussed in this section include the anaerobic contact process, the USB reactor, the anaerobic filter, and the AFBR.

Anaerobic contact process:

The anaerobic contact process is a suspended-growth process, similar in design to the activated-sludge process except that anaerobic conditions prevail in the former process. Figure 2 shows the process schematic. The anaerobic contact process is comprised of two parts. The contact part involves thorough mixing of the wastewater influent with a well-developed anaerobic sludge culture. The separation part involves the settling out of anaerobic sludge from the treated wastewater and recycling back to the contact reactor. The process usually has a vacuum degasifier placed following the aerobic reactor to eliminate gas bubbles that cause SS in the clarifier to float. BIOENERGYand ANAMET are two commercially available, proprietary anaerobic contact processes.

BIOENERGY is a conventional anaerobic contact process that uses a thermal shock procedure to facilitate sludge separation. As the mixed liquor, at 35°C, flows from the contact reactor to the settling unit, a series of heat exchangers rapidly decreases its temperature to 25°C. This temporarily interrupts gasification allowing effective sludge–solids separation by gravity. The temperature of the recycled sludge is increased before it is returned to the contact unit.

In the ANAMET process, an aerobic biological treatment polishing step follows the anaerobic contact process to provide near-complete organics removal. The process recycles the sludge produced in the aerobic treatment process back to the anaerobic reactor to reduce excess sludge production across the entire system and increase biogas yield. Also, recirculation of the nutrient-containing sludge from the aerobic reactor reduces external nutrient requirements in the anaerobic reactor.

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Figure 2: Anaerobic contact process [2]

USB reactor:

USB reactor is essentially a suspended-growth reactor, but it is also a fixed-biomass process. Figure 3 shows the process schematic. This USB system is based on the development of a sludge blanket. In this sludge blanket, the component particles are aggregated to withstand the hydraulic shear of the upwardly flowing wastewater without being carried upwards and out of the reactor. The sludge flocs must be structurally stable so that hydraulic shear forces do not break them into smaller portions that can be washed out, and they should also have good settlement properties. The wastewater is fed at the bottom of the reactor, and active anaerobic sludge solids convert the organics into methane and carbon dioxide. The anaerobic biomass is distributed over the sludge blanket and a granular sludge bed. The sludge solids concentration in the sludge bed is high—100,000 mg/l SS—and does not vary over a range of process conditions. The sludge solids concentration in the sludge blanket is lower and depends on process conditions. The reactor can include an internal baffle system, usually referred to as a gas–liquid separator, above the sludge blanket to separate the biogas, sludge, and liquid. A patented USB reactor called the BIOTHANE process was developed by the Biothane Corporation in the United States.

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Figure 3: Upflow sludge blanket reactor [2]

Anaerobic filter:

In an anaerobic filter reactor, the growth-supporting media is submerged in the wastewater. Anaerobic microorganisms grow on the media surface as well as inside the void spaces among the media particles. The media entraps the SS present in the influent wastewater that can be fed into the reactor from the bottom (upflow filter) or the top (downflow filter) as shown in the process schematics in Figure 4. Thus, the flow patterns in the filter can be either PF or completely mixed depending on recirculation magnitude. Periodically backwashing the filter solves bed-clogging and high-head-loss problems caused by the accumulation of biological and inert solids. BACARDI and CELROBIC are two proprietary anaerobic filter processes currently available.

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Figure 4: Anaerobic filter [2]

AFBR:

The AFBR is an expanded-bed reactor that retains media suspension from drag forces exerted by upflowing wastewater. Figure 5 shows the process schematic. Fluidization of the media particles provides a large surface area where biofilm formation and growth can occur.

The media particles have a high density resulting in a settling velocity that is high enough so that high-liquid-velocity conditions can be maintained in the reactor. However, the media particles’ overall density decreases as biomass growth accumulates on the surface area. The decrease in density can cause the bioparticles to rise and be washed out of the reactor. To prevent this situation, the reactor controls fluidized-bed height at a required level by wasting a corresponding amount of overgrown bioparticles. The wasted bioparticles can then be received by a mechanical device that separates the biomass from the wasted media particles. The cleaned particles can then be returned to the reactor, while the separated biomass is wasted as sludge.

The AFBR combines a suspended-growth system and an attached-growth system since biomass growth attaches to the media particles which are suspended in the wastewater. The reactor recycles a portion of the effluent flow ensuring uniform bed fluidization and sufficient substrate loading.

Some commercially available AFBR processes include the ANITRON system developed by Dorr-Oliver, Inc.; the BIOJET process, which employs an AFBR with an enlarged top section; and the ENSO-FENOX process, which combines an AFBR with a trickling filter. The AFBR has been applied to a variety of industrial treatment processes with substrates such as molasses, synthetic sucrose, sweet whey, whey permeate, glucose, and acid whey.

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Figure 5: Anaerobic fluidized bed reactor [2]

ADVANTAGES:

The major advantages of anaerobic treatment over aerobic treatment are as follows:

  1. The biomass yield for anaerobic processes is much lower than that for aerobic systems; thus, less biomass is produced per unit of organic material used. This reduced biomass means savings in excess sludge handling and disposal and lower nitrogen and phosphorus requirements.
  2. Since aeration is not required, capital costs and power consumption are lower.
  3. Methane gas produced in anaerobic processes provides an economically valuable end product.
  4. The savings from lower sludge production, electricity conservation, and methane production range from $0.20 to $0.50 per 1000 gal of domestic sewage treatment. The reduction of sludge and aeration energy consumption each result in savings that are greater than the cost of the energy required by the anaerobic process. In addition, a substantial part of the energy requirements for anaerobic processes can be obtained from exhaust gas.
  5. Higher influent organic loading is possible for anaerobic systems than for aerobic systems because the anaerobic process is not limited by the oxygen transfer capability at high-oxygen utilization rates in aerobic processes. [2]

DISADVANTAGES:

Some disadvantages are associated with the anaerobic process as follows:

  1. Energy is required by elevated reactor temperatures to maintain microbial activity at a practical rate. (Generally, the optimum temperature for anaerobic processes is 35°C.) This disadvantage is not serious if the methane gas produced by the process can supply the heat energy.
  2. Higher detention times are required for anaerobic processes than aerobic treatment. Thus, an economical treatment time can result in incomplete organic stabilization.
  3. Undesirable odors are produced in anaerobic processes due to the production of H2S gas and mercaptans. This limitation can be a problem in urban areas.
  4. Anaerobic biomass settling in the secondary clarifier is more difficult to treat than biomass sedimentation in the activated-sludge process. Therefore, the capital costs associated with clarification are higher.
  5. Operating anaerobic reactors is not as easy as aerobic units. Moreover, the anaerobic process is more sensitive to shock loads. [2]

COST INVOLVED IN ANAEROBIC TREATMENT:

Since aeration is not required, capital costs and power consumption are lower. Methane gas produced in anaerobic processes provides an economically valuable end product. The savings from lower sludge production, electricity conservation, and methane production range from $0.20 to $0.50 per 1000 gal of domestic sewage treatment. The reduction of sludge and aeration energy consumption each result in savings that are greater than the cost of the energy required by the anaerobic process. In addition, a substantial part of the energy requirements for anaerobic processes can be obtained from exhaust gas. [2]

REFERENCES:

  1. Dictionary of Water and Waste Management, Elsevier
  2. Environmental Engineers’ Handbook
  3. Wastewater treatment: biological and chemical processes By M. Henze
  4. Environmental Bioengineering By Lawrence K. Wang, Joo-Hwa Tay, Stephen Tiong Lee Tay
  5. Biological wastewater treatment: principles, modelling and design By M. Henze

Last Edited: January 13 2018

Waste management

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Volunteer Image Author Prylarer

Before we begin with the technical parts of waste management, I would like to share an article in which Dan McDougall reports from the chaos and filth of Dharavi in Mumbai, where a recycling industry is helping thousands claw a way out of poverty. Find the article here: Waste not, want not in the £700m slum. We can see how waste can be managed with such ingenuity.

Waste management can be broadly divided into three sections:

  1. Liquid waste management
  2. Gaseous waste management
  3. Solid waste management

Waste streams can be managed by processes like:

  • Extraction
  • Oxidation
  • Hydroprocessing

How is it done?

  • Waste is first analyzed for valuable products
  • Valuable products are recovered by the processes mentioned above
  • When solvent is used which recovery, it should:
    • not be soluble in water
    • safe to handle
    • biodegradable
    • safe for humans
    • low cost

Extraction:

Speaking of extraction, it can be:

  • Liquid-liquid extraction (physical or chemical)
  • Liquid-solid extraction (aka adsorption)

Coming to adsorption, here are some examples of commercial interest:

  • Polymeric adsorbents
    • Example: Phenol Amberlite™ XAD™4 polymeric adsorbent is used in several locations around the world to remove phenol from wastewater. Even high concentrations of phenol (20,000 ppm) in wastewater have been effectively treated. The resin’s capacity for phenol increases with increasing phenol concentration. Regeneration of the resin is accomplished in several ways: 1% caustic or solvents such as acetone, methanol and formaldehyde. Acetone is frequently used since most phenol plants also have acetone production.
  • Activated carbon bed
  • Zeolites: These are microporous aluminosilicate minerals

Hydroprocessing:

Hydroprocessing can be divided into three categories:

  1. Hydrogenation
  2. Hydrocracking
  3. Hydrotreating

Hydroprocessing of nitro compounds:

Nitro compounds can be converted into amino compounds by catalytic transfer hydrogenation (CTH). Catalyst is by and large Pd/C and ammonium formate acts as a hydrogen donor. This is done at a temperature of 80 deg C.

Ammonia formate decomposes to nascent hydrogen and gives out NH3. If you use sodium formate, you’ll get a precipitate which is not desirable. The amino compounds can be taken for wet oxidation, direct use of nitro compounds can cause explosions.

A bit about CTH:

Reduction of organic functional groups can be categorized into (i) addition of hydrogen to unsaturated groups as, for example, in the reduction of ketones to alcohols and (ii) addition of hydrogen across single bonds leading to cleavage of functional groups (hydrogenolysis). Molecular hydrogen, a gas of low molecular weight and therefore high diffusibility, is easily ignited and presents considerable hazards, particularly on the large scale; the use of hydrogen donors obviates these difficulties in that no gas containment is necessary, no pressure vessels are needed, and simple stirring of solutions is usually all that is required.

Hydrogenation of organic molecules is one of the processes most used in the synthetic organic chemistry industry. The most common methodology for this process is catalytic hydrogenation, either under homogeneous or heterogeneous catalysis, involving molecular hydrogen and a transition metal. However, despite being a reaction of proven efficiency, it presents a large drawback related with the handling of hydrogen gas (flammable and explosive). For this reason, the scientific community has been working hard in catalytic transfer hydrogenation, which avoids the use of molecular hydrogen in favor of hydrogen donors (alcohols, diimides, amines, hydrocarbons or formic acid).

Did I miss anything? Ah, yes. A very primitive method of waste management. It is called Incineration. But there are several problems associated with this method and hence is not the preferred one.

Disadvantages of Incineration:

  • It causes evolution of HCl
  • In presence of aromatics, dioxins are produced. Dioxins are toxic.

Hydroprocessing of halogenated aromatics:

In case of chlorinated aromatics, HCl evolves. This acid needs to be neutralized and is done with NaOH.

HCl + NaOH –> NaCl +H2O

The catalyst used for this purpose usually is Pd since it is famous for dehalogenation reactions while Ru is known for benzene ring saturation.

Did you know? Ru works wonders when in water than alone. No one knows why.

Oxidation:

Oxidation can be biological (aerobic) or chemical (molecular O2 via OH* radicals). Wet air oxidation is one other way of handling waste.  It is highly capital intensive as compared to incineration.

Did you know? Anaerobic processes create methane. Methane gives energy.

Read more:

Wet air oxidation by Prof. V. V. Mahajani