Diffusion and Advection

DIFFUSION and ADVECTION

Diffusion

Diffusion refers to the movement of a substance from an area of high concentration to an area of low concentration. Diffusion is driven by a gradient in the chemical potential of the diffusing species (If one atom or one molecule is identical to another, the are said to belong to the same species). The two main types of diffusion are passive diffusion and facilitated diffusion.

Active diffusion involves the presence of an additional energy to transport a substance from an area of high concentration to an area of low concentration. Active diffusion is also called facilitated diffusion. Passive diffusion does not require the any additional energy to transport a substance across a membrane.

Advection

Advection is similar to convection. It refers to horizontal movement of the solute with the bulk solvent in a macroscopic sense. This includes transport of pollutants.

Diffusion and Advection in wastewater treatment

Transport processes in the environment may be divided into advection and diffusion. Advection refers to transport with the mean fluid flow. For example, if the wind is blowing toward the east, advection will carry any pollutants present in the atmosphere toward the east. Similarly, if a bag of colour is emptied into the center of a river, advection will carry the resulting colour downstream. 

• Diffusion refers to the transport of compounds through the action of random motions.
• Diffusion works to eliminate sharp discontinuities in concentration and results in smoother, flatter concentration profiles. 

Advective and diffusive processes can usually be considered independently.
In the example of a colour in a river, while advection moves the center of mass of the colour downstream, diffusion spreads out the concentrated colour to a larger, less concentrated region.

mass transport mathematics

MATHEMATICS OF MASS TRANSPORT

• Mass transport refers to the movement of substances from one point to another involving movement with the fluid and dispersion or diffusion. Various computer models are used to predict the fate of pollutants and their effects on the ecosystem.
• Mass balance is a very important tool to track pollutants in the environment and for the design of a treatment component.
• The basis for mass balance is "Law of conservation of mass"
• Mass balance is performed over specific control volumes having well defined system boundaries
• Control Volume (CV) is a specific region in space for which mass balance is written. It defines mass flow rates into and out of the system
• Mass enters and exits the control volume
• There may be an increase or decrease in the amount of mass in the control volume due to physical, chemical and or biological reactions
• Overall mass balance between times t and t+Δt = (mass at time t) + (mass entered system between t and t+Δt) - (mass exited between t and t+Δt) + (mass generated or consumed by reaction processes between t and t+Δt)
• Overall mass accumulation (ΔM/Δt) between times (t and t+Δt) is given by:

((mass at time t+Δt) - (mass at time t)) / Δt

=

(mass entered system between t and (t+Δt)) / Δt

-

(mass exited between t and (t+Δt)) / Δt

+

(mass generated or consumed by processes between t and (t+Δt)) / Δt

 As Δt approaches 0 the overall rate of mass accumulation:

(rate of mass accumulation) = (rate mass influx) - (rate mass outflux) + (rate net mass production or consumption)

dM/dt = dMin/dt - dMout/dt + dMrxn/dt

Mass accumulation term = dm/dt

For:

Steady state:       dm/dt = 0

Unsteady state:   dm/dt not equal to zero

Rate of mass in = dm (in)/dt = Qin * Cin

Rate of mass out = dm (out)/dt = Qout * Cout

Balance for a simple constant volume CSTR:

V dc/dt = Qin * Cin - Qout * Cout

Consider a steady state mass balance on a lake only that has two rivers feeding into it and one river flowing out. Assuming that the lake is well-mixed, it behaves like a CSTR

Considering a steady-state mass balance on the lake water only:

Rivers feeding IN:

C1(in) = 0 mg/L, Q1(in) = 5000 L/min

C2(in) = 0 mg/L, Q2(in) = 1000 L/min

One river flowing OUT:

C(out), Q(out) = Q1(in) + Q2(in) = 6000 L/min

At steady-state, there is no net accumulation or depletion of water in the lake

Consider mass balance on  the polluted lake:

C1(in) = 20 mg/L                                                  C1 carries the pollutant, 'X'
Q1(in) = 5000 L/min
C2(in) = 0 mg/L
Q2(in) = 1000 L/min

C(out) = 6000 L/min
Steady state conditions imply that amount of pollutant and water that flows in the lake should be equal to the amount of pollutant and water that flows out.
Carrying out a mass balance:

C(out)*Q(out) = [C1(in) * Q1(in)] + [ C2(in)+ Q2(IN)]

C(out) * 6000 L/min = [(20 mg/L * 5000 L/min) + (0 mg/L * 1000 L/min)]

Therefore:

C(out) = 20 * 5000/6000 = 16.7 mg/L

This implies that the concentration of the pollutant is diluted to 
16.7 mg/L in the river flowing out

disposal into lakes and rivers

Wastewater disposal to water environments

Disposal into a lake, stream or ocean needs to take into account the ability of the receiving water to assimilate wastewater. The natural purification capacity of the environment is limited. Even when wastewater is disposed to the ocean, the area surrounding the outfall can be sufficiently polluted and the pollutants (including pathogens) can be washed towards the beaches. Nutrients (nitrogen and phosphorus) promote the growth of algae in the receiving water. In lakes and sensitive water environments the removal of nutrients may be required. Furthermore if the wastewater contains high levels of heavy metals and toxic chemicals, these may have to be removed before wastewater disposal. Over the years the requirement for disposal into water environments has become stricter.

Wastewater Disposal

There are three methods by which final disposal of wastewater can be accomplished. The general problem areas that are of concern in final disposal are pathogenic microorganisms (viruses, etc.), heavy metals and the presence of biologically resistant organic compounds, such as pesticides or insecticides which can find their way into water supplies. 


Surface Disposal

Generally this is disposal by irrigation. This involves spreading the wastewater over the surface of the ground, generally by irrigation ditches. There is some evaporation, but most of the wastewater soaks into the ground and supplies moisture with small amounts of fertilizing ingredients for plant life. This method is largely restricted to small volumes of wastewater from a relatively small population where land area is available and where nuisance problems will not be created. It has its best use in arid or semi-arid areas where the moisture added to the soil is of special value. If crops are cultivated on the disposal area, the growth of vegetation, often must be excluded from wastewater. Because untreated wastewater will also contain pathogenic organisms, the production of foods for human consumption which may be eaten without cooking is not desirable.

Subsurface Disposal

By this method wastewater is introduced into the ground below its surface through pits or tile fields. It is commonly used for disposal of settled wastewater from residences or institutions where there is only a limited volume of wastewater.
                                                              Disposal by Dilution

Disposal by dilution is the simple method of discharging wastewater into a surface water such as a river, lake, ocean, estuaries or wetlands. This results in the pollution of the receiving water. The degree of pollution depends on the dilution, volume and composition of the wastewater as compared to the volume and quality of the water with which it is mixed. When the volume and organic content of the wastewater is small, compared with the volume of the receiving water, the dissolved oxygen present in the receiving water is adequate to provide for aerobic decomposition of the organic solids in the wastewater so that nuisance conditions do not develop. 

Where the dissolved oxygen in the receiving water is inadequate to maintain aerobic decomposition, anaerobic decomposition takes place and putrefaction with objectionable conditions results. A volume of of wastewater that has been treated to remove or reduce this organic matter can be discharged to a natural surface water. The dissolved oxygen in the receiving water is an important factor for aerobic decomposition of organic waste.

The problems associated with this type of disposal are the effects of toxic or potentially toxic compounds found in domestic and industrial wastewater. These may involve immediate toxic effects such as heavy metals in fish and the "concentration" of certain biologically resistant compounds in the food chain. An example would be the accumulation of certain pesticides by microorganisms that are consumed by higher organisms to include fish, birds, and even man. Another environmental effect of concern due to disposal of untreated wastewater by dilution is the enrichment of receiving waters by the introduction of plant nutrients such as nitrogen and phosphorous. The presence of excessive amounts of these nutrients can stimulate plant and algae growth in the receiving waters.

Need for Wastewater Treatment

The problem of wastewater disposal developed with man's use of water as a vehicle for carrying away the waste products of human life. Prior to that the volume of wastes, without the water vehicle, was small and disposal was largely restricted to the individual's or family's excreta. The earliest practice was simply to leave body waste and garbage on the surface of the ground where it was gradually decayed by bacteria, mostly the saprophytic anaerobic type. This caused the production of foul odors. Later, experience showed that if these wastes were promptly buried the odors could no longer be detected. Burial of human waste is a very ancient practice and even has biblical references. The next logical step was the development of the earth privy or outhouse, a method for the disposal of excremental wastes which is still widely used.

With urbanization and the development of community water supplies and the use of water to flush or transport wastes from habitations, it became necessary to find disposal methods not only for the wastes themselves, but for the water which carried them. All of the three possible methods - irrigation, subsurface disposal and dilution - were employed.

As urban communities increased in population, with proportional increase in the volume of wastewater and in the amount of organic waste, all methods of disposal resulted in such unsatisfactory conditions that remedial measures became essential and the development of methods of treatment of wastewaters prior to ultimate disposal was started.

The objectives originally sought in wastewater treatment include :

- Protection and maintenance of sources for use as domestic water supplies.

- Prevention of disease and spread of diseases.

- Prevention of nuisance conditions.

- Maintenance of clean waters for bathing and other recreational purposes.

- Protection and maintenance of the environment. For example, maintaining natural waters for the propagation and survival of fish life.

- Conservation and protection of water for industrial and agricultural uses.

- Prevention of silting in navigable channels.

A wastewater treatment plant is designed to remove from the wastewater enough organic and inorganic solids so that it can be disposed of without contravening or affecting the objectives sought.

Treatment devices merely localize and confine these processes to a restricted, controlled, suitable area or environment and provide favorable conditions for the acceleration of the physical and biochemical reactions. The extent or degree of treatment needed varies greatly from place to place and is regulated by law. In general, the following are the determining factors :

- The character and amount of the solids carried by the wastewater, that is; BOD and suspended solids present.

- The objectives sought.

- The ability or capacity of the land (in disposal by irrigation and subsurface disposal) or the receiving water (in disposal by dilution) to handle by self-purification or dispersal the water and solids in the wastewaters.

- Legal aspects and constraints.

The degree of wastewater treatment required to satisfy the first three conditions above is variable and is highly dependent on the local conditions and needs. Simple settling or even the mere removal of floating solids by screens may be adequate for wastewaters under certain conditions, while a very high removal of suspended solids, decomposition of dissolved organic solids and destruction of pathogenic organisms may be required before discharge to a river which is used downstream as a source of public water supply.

After the disposal of the wastewater effluent from a treatment plant, there still remains in the plant the solids and water constituting the sludge which has been removed from the wastewater. This too must be disposed of safely and without nuisance.

The progress of self-purification of a stream can be measured by appropriate physical, chemical and biological laboratory tests. Similar tests are used to measure and control the progress of wastewater treatment plant processes.

The serious problem involving the disposal of wastewaters and other wastes by adequate and effective means that will eliminate nuisances and not violate the rights and welfare of individuals and communities has led to the development of laws and regulations governing such disposal.

It is presumed that in ancient times, customs slowly developed which regulated the disposal of the wastes of the individuals and of the group. As time went on, custom took on the force of law and led, over the years, to the formulation of legal regulations - first as common law and then as statutory law.

Disposal

Satisfactory disposal of wastewater, whether by surface, subsurface methods or dilution, is dependent on its treatment prior to disposal. Adequate treatment is necessary to prevent contamination of receiving waters to a degree which might interfere with their best or intended use, whether it be for water supply, recreation, or any other required purpose.

Wastewater treatment consists of applying known technology to improve or upgrade the quality of a wastewater. Usually wastewater treatment will involve collecting the wastewater in a central, segregated location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment processes. Most often, since large volumes of wastewater are involved, treatment processes are carried out on continuously flowing wastewaters (continuous flow or "open" systems) rather than as "batch" or a series of periodic treatment processes in which treatment is carried out on parcels or "batches" of wastewaters. While most wastewater treatment processes are continuous flow, certain operations, such as vacuum filtration, involving as it does, storage of sludge, the addition of chemicals, filtration and removal or disposal of the treated sludge, are routinely handled as periodic batch operations.

receiving water standards

RECEIVING WATER STANDARDS

In the context of wastewater treatment, the term, 'receiving water ' is defined as a stream, river, lake, ocean or other body into which wastewater or treated effluent is discharged. The Central Pollution Control Board (CPCB) under the environment protection rules of 1986 in schedule VI, has set "General Standards for discharge of environmental pollutants" as effluents as given below:

1. Colour and odour 
2. Suspended solids not to exceed 100 mg/l
3. Particulate size of suspended particles should pass through 850 micron IS sieve
4. pH value should lie between 5.5 and 9
5. Temperature should not exceed 5 C more than receiving water temperature
6. Oil and grease should not exceed 10 mg/l
7. Total residual chlorine should not exceed 1 mg/l
8. Ammonical nitrogen as (N) should not exceed 50 mg/l
9. Total kjeldahl as ammonia should not exceed 100 mg/l
10. Free Ammonia should not exceed 5 mg/l
11. BOD for 3 days at 27C should not exceed 30 mg/l
12. COD should not exceed 250 mg/l
13. Arsenic should not exceed 0.2 mg/l
14. Mercury should not exceed 0.01 mg/l
15. Lead should not exceed 1 mg/l
16. Cadmium should not exceed 2 mg/l
17. Hexavalent chromium (Cr6+) should not exceed 0.1 mg/l
18. Copper should not exceed should not exceed 3 mg/l
19. Zinc should not exceed 5 mg/l
20. Selenium should not exceed 0.05 mg/l
21. Nickel should not exceed 3 mg/l
22. Cyanide should not exceed 0.2 mg/l
23. Fluoride should not exceed 2.0 should not exceed 
24. Sulphides should not exceed 2.0 mg/l
25. Dissolved Phosphates should not exceed 5.0 mg/l
26. Phenoile compounds should not exceed 1 mg/l
27. Radioactive materials
1. Alpha emitter should not exceed  10^-7 mg/l
2. Beta emitter should not exceed 10^-6 mg/l
28. Manganese should not exceed 2 mg/l
29. Iron should not exceed 3 mg/l
30. Vanadium should not exceed 0.2 mg/l
31. Nitrate Nitrogen should not exceed 10 mg/l

Sludge calculations

DAILY SLUDGE PRODUCTION OF WASTEWATER TREATMENT PLANT

Terms used & their notation

• Q  -      Wastewater flow rate (m3/d) (Influent flowrate)
• So  -      Influent Soluble Substrate Concentration (BOD or bsCOD g/m3)
• S   -      Effluent Soluble Substrate Concentration
• Xo,i -   nbVSS concentration in effluent (g/m3 or mg/l)
• iTSS -   inert inorganic Total Suspended Solids (g/m3)
• XT    -  Total MLVSS concentration (g/m3 or mg/l)
• SRT  -   Sedimentation Retention Time (d)
• VSSo - Influent Volatile Suspended Solids
• Px,T, TSS - Daily Sludge production kg TSS/d
  Px,T, VSS - Daily Sludge production kg VSS/d
• Kinetic coefficients
• k - maximum rate of soluble substrate utilization (g COD/g . d)
• y - biomass yield (g VSS/g COD)
• kd - endogenous decay coefficient (g VSS/g VSS . d)
• ks - half velocity constant (g COD/m3)
• fd - fraction of cell mass that remains as cell debries (g VSS/g VSS)

CASE STUDY

• Q = 1000 m3/d
• So = 192 BOD or bsCOD g/m3
• Xo, i = 30 g/m3
• iTSS = 10 g.m3
• XT = 2500 g/m3
• SRT = 6 d
• k = 12.5 g COD/g . d
• y = 0.4 g VSS/g COD
• kd = 0.1 g VSS/G VSS . d
• ks = 1 g COD/m3
• fd = 0.15 g VSS/g VSS

DAILY SLUDGE PRODUCTION 

Px, T, VSS = 82.17  kg VSS/d 

Px, T, TSS = 101.37 kg TSS/d

Useful information regarding sludge calculation

Outfall design

OUTFALL DESIGN

An outfall is an underwater pipeline that discharges wastewater into receiving water. Even if the wastewater has received extensive treatment, it is desirable to dispose the effluent to minimize possible impairment to quality of receiving waters near point of discharge by using a diffuser. 

A diffuser is a section of the outfall, usually deepest and most seaward portion with small holes or ports along its length. These ports discharge wastewater in small quantities as opposed to the entire flow being discharged at one point.

Diffuser ports may be simple holes in the outfall pipe or short tubes extending from the pipe. The type of port used depends upon particular conditions of final installation. The various elements of outfall design process are:

• Site selection
• Outfall hydraulics
• Dilution and Mixing
• Diffuser port design
• Pipe design
• Pipe support systems
• Construction methods

The design is phased into four elements:

1. Feasibility study
2. Pre-design
3. Preliminary design and
4. Final design

The flow diagram illustrating how various design components and phases are integrated is depicted in the chart below:

Site selection:

• Discharge port for an outfall should be located in proximity to its companion wastewater treatment plant.
• First step in outfall design is to determine feasibility of linking treatment plant to nearest receiving water.
• This involves study of:
• Topographic maps
• Hydrographic charts
• Literature of coastal processes like tides, waves, currents and geology
• Characteristics of the site and obtaining related water quality data related to proposed receiving water
• If the information suggests that, from the construction cost and water quality point of view one or more outfall routes is feasible, the pre-design engineering studies are commenced to select a final route or an alternative disposal system must be considered.

The following outfall siting criteria are considered in detail in the pre-design stage:

• Bottom topography and turf zone
• Physical oceanography
• Water quality and
• Wastewater soils and geology

A hydrographic survey of the general area of the proposed outfall alignment is performed using an electronic recording echo-sounding device combined with horizontal positioning system. Helicopters may be used as sounding platforms. Subsequently, profiles of bottom topography are plotted. Analysis of profiles will reveal potential outfall routes. It is desirable to have a continually declining grade for the outfall line. This prevents potential sludge build-up in low parts and accumulation of air at high points which can reduce the hydraulic capacity of the outfall. There is no ideal slope for locating the diffuser. A relatively flat slope is desirable. The diffuser port should be designed such that there should be equal discharge from  each side of the diffuser. This results in equal dilution. A mild slope is beneficial for achieving uniform port discharge. 

Penetrating the surf zone is most expensive and difficult phase of constructing an ocean outfall because of continuous wave attack in shallow water.

Sporadic storms....

Important terms explained

IMPORTANT TERMS

• Unit processes
• Unit process refers to chemical conversion of reactants into desirable products. Ex: Oxidation, Nitration, Hydrogenation, desulphurization of sulphur, etc
• In unit process there is no physical change but chemical changes occur
• Unit process is secondary
• Unit process involve both physical and chemical changes
• Unit process is a chemical conversion of reactants into desired products.

• Unit operations
• Unit operations refer to separation of products by physical means.    Ex: Absorption, Adsorption, Distillation, Membrane separation, mixing, agitation, drying, quenching, evaporation, crushing, grinding, crystallization,  separation, etc
• Each step is one unit operation and the entire process is unit process
• In unit operations, no chemical changes but physical changes occur
• Unit operations is a physical change
• Unit operations need to be performed in unit process
• Unit operations involve physical changes only
• Unit operation is separation of products by physical means

Both unit processes and unit operations are transformatory activities

• Biological unit process
•  Biological unit process make use of microbes for removal of dissolved organic matter from wastewater. The two types of biological unit processes are:
• Aerobic biological unit process and
• Anaerobic biological unit process
• Biological unit processes carry out treatment of wastewater by microorganisms in the presence (aerobic) or absence of oxygen (anaerobic)
• Examples of aerobic processes are:
• Activated Sludge Process
• Trickling filters
• Aerobic stabilization ponds and
• Aerated lagoons
• Rotating Biological Contactors
• Oxidation ponds
•  Examples of anaerobic processes are:
• Anaerobic sludge digestion
• Anaerobic contact process
• Anaerobic filters
• Anaerobic lagoons or ponds

• Treatment system
• The type of combination used from the available unit operations and processes for treatment of a particular waste is known as treatment system.
• Primary treatment system
• As per the ongoing technology, the wastewater treatment system that includes all units of the preliminary treatment system such as 'sump and pump house' , 'approach channel', 'bar screen', 'grit chamber', 'parshall flume or velocity control device', 'skimming tank' and the 'primary sedimentation tank' together are known as the primary treatment system
• Secondary treatment system
• The removal of colloidal and soluble organic content is carried out in the secondary treatment system consisting of an aeration basin with return sludge facility or a trickling filter and a secondary sedimentation tank is called a secondary treatment facility
• Waste stabilization ponds (Oxidation ponds)
•  Waste stabilization ponds are large, man-made water bodies in which black water or grey water or fecal sludge are treated by naturally occurring processes under the influence of sunlight, wind, microorganisms and algae. They may be aerobic, anaerobic or facultative each having different treatment and design characteristics. These ponds are low on operation and maintenance and have high removal rate for pathogens and BOD. They require large surface areas and special expertise in design of such systems. The effluent contains high amount of Nitrogen and Phosphorus and hence can be used for agricultural purposes but not for recharge of groundwater.
• Oxidation lagoons (Aerated lagoons)
•  Oxidation lagoons or aerated lagoons operate on the principle of the activated sludge process
• It consists of an earthen basin in which wastewater is fed only after screening
• The wastewater is treated with or without recycling of sludge
• Oxygen required is provided by surface aerators or submerged diffuse aeration system
• The system assumes complete mixing and the flow of wastewater is continuous
• Oxidation ditches (Extended Aeration Systems)
• An oxidation ditch is a modified activated sludge biological treatment process that utilises long solids retention time to remove biodegradable organics. Oxidation ditches are typically complete mix systems.
• Oxidation ditches are suitable for areas where land availability is high.
• They are easy to maintain and adapt easily to shock loads.
• Rotating Biological Contactor (RBC)
• Rotating Biological Contactors also known as Rotating Biological Filters are fixed bed reactors consisting of stacks of rotating discs mounted on a horizontal shaft. They are partially submerged and rotated as wastewater flows through. The microorganisms are alternately exposed to atmosphere and wastewater, allowing both aeration and assimilation of dissolved organic pollutants and nutrients thereby degrading the pollutants
• Up-flow Anaerobic Filter (UAF)
•  An Up-flow Anaerobic Filter (UAF) is characterised by long detention time and production of anaerobic effluent. It uses physical removal mechanisms like flocculation, sedimentation and absorption. Anaerobic digestion also occurs in the bed. The bed is generally filled with gravel.
• Up-flow Anaerobic Sludge Blanket
• Up-flow Anaerobic Filter is also known as  Up-flow Anaerobic Sludge Blanket Reactor. In this treatment process, a blanket of biologically formed sludge granules act as a filter through which the wastewater is passed where solid-liquid separation takes place. The separated settling solids return to the active blanket zone while the liquid passes over the weir as effluent. The waste solids get attached to the biomass  and are stabilized. The gas produced as a result of stabilization is collected.
• This process is an important option for sewage treatment in warm countries
• Tertiary or Advanced Treatment System
• Tertiary treatment is the final cleaning process that improves wastewater quality before it is reused, recycled or discharged to the environment. The treatment removes remaining inorganic compounds and substances such as Nitrogen and Phosphorus
• Bacteria, viruses and parasites which are harmful to public health are removed in this stage
• Alum is used to remove additional phosphorus and group the remaining solids together for easy removal in the filters
• Due to addition of alum, tiny particles cluster together in masses called floc.
• Filters are back-washed every 24 hours to remove accumulated floc.
• Chlorine contact tank removes microorganisms in tertiary treated wastewater
• Remaining chlorine is removed by adding sodium bisulphite
• Granular media filtration
• Any process that removes suspended particles through a porous medium is defined as filtration
• Granular Media Filters can be:
• Slow sand filters
• Rapid sand filters 
• Granular media types:
• Mono media - Silica sand
• Dual media - Anthracite coal or Granular Activated Carbon
• Multimedia - Garnet
• Ultrafiltration
•  Ultrafiltration is a type of membrane filtration in which forces like pressure or concentration gradients lead to separation through a semipermeable membrane.
• Suspended solids and solutes of higher molecular weight are retained on the retentate while water and low molecular weight solutes pass through the membrane in the permeate.
• Ultrafiltration can be used to remove particulates and macro-molecules from raw water to produce potable water.
• It can replace secondary or tertiary treatment in conventional water treatment systems or can be used as a standalone system
• It is sometimes integrated in water treatment systems in the pre-treatment system
• After ultrafiltration, the recycled water can be reused for a number of industrial purposes like boiler or cooling tower feed, water supplementation, pH adjustment, washing equipment and vehicles, fire protection, process rinse water for production lines, toilet flushing, dust control, construction activities and concrete mixing
• Industries that consume large volumes of water or discharge toxic effluents employ ultrafiltration for reusing water
• Ultrafiltration is frequently used to pretreat surface water, sea water and biologically treated municipal water ahead of reverse osmosis unit
• Micro-strainers
•  
• Biological nitrification/denitrification
• Ion exchange
• Air stripping
• Chemical processes
• Reverse osmosis
• Electrodialysis
• Chemical precipitation
• Adsorption
• Treatment process vs Treatment system
• Population projection
• Flow rates and their fluctuations
• Mass loading
• Design criteria
• Hydraulic flow diagram
• Total solids, suspended solids, Total dissolved solids, Volatile solids, Fixed solids, Miineral solids
• BOD
• COD
• Surfactants
• Oil and grease
• Alkalinity
• Chlorides
• Total Nitrogen as N
• Sulphur
• Total Phosphorus as P
• Alkalinity as CaCO3
• Heavy metals
• Strength of wastewater
• Design period
• Flow variations
• Daily flow rate
• Average daily flow rate
• Maximum daily flow rate
• Minimum daily flow rate
• Dry Weather Flow (DWF)
• Concept of mass load
• Detention period or Hydraulic Retention Time
• Flow through velocity
• Settling velocity
• Hydraulic Loading or Surface Loading Rate or Over Flow Rate
• Weir Loading Rate
• Volumetric Loading or Organic Loading (BOD or COD or VSS loading)
• Food to Microorganism ratio (F/M) 
• The ratio of incoming BOD to MLSS, the F/M ratio is also known as the loading on the system and is calculated as kg of BOD per day per kg of MLSS in the aeration tank.
   
• If the F/M ratio is low and the aeration period or detention time (in the aeration tank) is long, the microorganisms make maximum use of available food, resulting in a high degree of treatment. Such systems are known as extended aeration systems and are mostly used for isolated sources (small developments).
   
• Mean Cell Residence Time or Solids Retention time
• Basin Geometry (L:B:D for rectangular tanks) or (Diameter & side water depth for circular tanks)
• Reactor
• Sizing of units
• Operation units are normally designed at peak flows
• Determination of surface area
• Determination of cross-sectional area
• Stoichiometry
• Rate of reaction
• Homogeneous reaction
• Heterogeneous reaction
• Rate of reaction
• Effect of temperature on reactions
• Treatment units (Reactors)
• Types of reactors
• CFSTR
• PFR
• BR
• AFR
• FBR
• PBR
• SBR
• Reactors in series
• Primary settling tank; Types of settling
• Concept of biological treatment
• Aerobic process
• Anaerobic process
• Facultative process
• Anoxic process
• Suspended growth process
• Attached growth or fixed film process
• Bio-kinetic coefficients (Growth constants)
• Microbial growth rate
• Substrate utilization rate
• Limiting substrate
• Endogenous decay
• Significant bio-kinetic coefficients
• Specific growth rate
• Yield coefficient
• Maximum Substrate Utilization Rate Constant
• Half velocity constant
• Endogenous decay coefficient
•  Reactor biomass and biomass yield
• BOD rate constant
• CODtotal
• CODb = 1.6 * BOD
• BODu/BOD5 = 1.5 (for domestic wastewater)
• Organic loading
• Quantity of air required
• Diffuse aeration system
• Surface aeration system
• Power required for oxygenation
• Volatile Suspended Solids (VSS)
• Mixed Liquor Suspended Solids (MLSS)
• The combination of the liquid and microorganisms undergoing aeration is known as mixed liquor, and the suspended solids are called Mixed Liquor Suspended Solids (MLSS).
  
   
• Mixed Liquor Volatile Suspended Solids (MLVSS) - MLVSS/MLSS = 0.8
• Sludge Volume Index (SVI)
• Sludge wasting
• ----

The Hydraulic Retention Time is the average time the liquid remains in the reactor while the Solids Retention Time (SRT) is the average time the solids remain in the reactor. SRT is also known as Sludge Age or Mean Cell Residence Time.

In extended aeration ponds have a diverse ecology thereby creating little to no biomass leading to savings in operational costs.

Overland flow systems

Overland flow systems 

 

In overland flow, wastewater is sprayed onto an inclined vegetated terrace and slowly flows to a collection ditch. Purification is achieved by physical, chemical, and biological processes, and the collected water is usually discharged into a nearby stream.

• Overland flow systems are used to achieve secondary treatment effluent quality for effluents coming from primary treatment facilities.
• Overland flow systems ensure high removal of Nitrogen(N), Suspended Solids(SS) and Biochemical Oxygen Demand (BOD)
• Overland flow systems apply previously treated wastewater to a vegetation covered graded land
• Application is done by grated pipes or nozzles at top of slope or by sprinkler systems within the site
• It is best suited for impermeable soils
• Overland flow systems require:
• low permeability soils
• grading within 2 - 8%

• Overland flow systems use the following mechanisms for removal of BOD and SS:
• Biological oxidation
• Sedimentation
• Filtration

• 75 - 90% of Nitrogen is removed by:
• Plant uptake
• Denitrification
• Ammonia volatalization

• 70 - 90% of Phosphorus can be removed by addition of alum of ferric chloride prior to land application. Phosphorus is removed by:
• Fixation processes in soil matrix
• Crop uptake

Effluent is collected in ditches and can be reused or discharged to a surface water body.

The treatment goal of overland flow systems is secondary treatment of wastewater and Nitrogen removal. In order to achieve these objectives the system requires a warm season. At the end of overland flow systems the effluent will have following characteristics:

• BOD < 10 mg/L
• TSS < 10 mg/L
• Total Nitrogen < 10 mg/L 

Rapid infiltration systems

RAPID INFILTRATION SYSTEMS

• In the rapid infiltration method, the wastewater is in large ponds called recharge basins. Most of it percolates to the groundwater, and very little is absorbed by vegetation. 
• To employ this method for treatment of wastewater, soils must be highly permeable. 
• Rapid Infiltration (RI) is also called soil aquifer treatment. 
• RI uses the soil ecosystem to treat wastewater. It can treat a large volume of wastewater on a much smaller area of land than other land treatment systems. 
• In RI systems, wastewater is applied to shallow basins constructed in deep and permeable deposits of highly porous soils. 
• Wastewater application can be by flooding or by sprinklers. 

• Treatment of wastewater occurs by
•  filtration
• adsorption
• ion exchange
• precipitation and 
• microbial action as the wastewater moves through the soil matrix.
• Phosphorus and most metals are retained in the soil while toxic organics are degraded or adsorbed.

• As wastewater percolates through the soil, it can be collected or it can flow to native surface water or groundwater aquifers. 
• If the groundwater table is relatively shallow, underdrains can be used to control groundwater mounding and recovery of renovated water.
• The recovered water can be used for irrigating crops or for industrial use (beneficial uses).
• Water that is not recovered can be used for recharging groundwater aquifers. 
• A major cause of concern is increased nitrogen levels in aquifers surrounding RI systems. 
• In order to address these concerns, the following modifications have been suggested:
• RI sites may be located next to rivers or other surface water bodies provided the percolate flows to surface water body and not affect the general groundwater quality
• Designing the system in such a way that the discharge rate only slightly exceeds the percolation rate thereby preventing adverse impact on the surrounding groundwater
• Maximizing nitrification/de-nitrification reactions
• Recycling percolate with maximum nitrate concenration.

Advantages

1. RI is a simple and low cost wastewater treatment concept that has been in use for more than a century
2. It can be used for treatment of both primary and secondary effluent from domestic and industrial wastewater
3. Industries such a breweries, distilleries, food processing plants, paper mills and wool scouring plants use RI process for treatment of their effluent.
4. This technique can be used in a variety of climates and different site locations.
5. RI process do not need any special seasons for effective implementation
6. Unless groundwater recharge and recovery is required, desirable sites are located immediately adjacent to surface waters thereby minimizing impact on general groundwater quality
7. An underdrain system can be located wherever suitable soil and groundwater conditions exist.
8. It is a gravity distribution method and consumes no energy
9. No chemicals are required
10. It is not affected by seasonal changes
11. Effluent is of excellent quality
12. It is a reliable process with sufficient resting periods
13. It can be used for groundwater recharge, controlling groundwater levels, recovering renovated water for reuse or discharge to a particular surface water body and temporary storage of renovated water in the aquifer.
14. The process is suitable for small plants where operator expertise is limited.

Disadvantages

1. RI systems do not meet the stringent nitrogen levels required for discharge to drinking water aquifers
2. Requires long term commitment of land area for treatment with minimal secondary benefits
3. Requires annual removal of accumulated deposits of organic matter on infiltration surfaces in the basins
4. May require occasional removal and disposal of top few inches of soil to expose clean material
5. Clogging can occur when influent is received at high application rates from algal laden facultative lagoons and polishing ponds.

Design Criteria

1. Primary design criteria for an RI system is site selection
2. To ensure a successful design, the important factors to be evaluated are:
   1. Soil depth
   2. Soil permeability and
   3. Depth to groundwater

The other factors to be considered after selecting a suitable site are:

1. Hydraulic loading rate
2. Nitrogen loading rate
3. Organic loading rate
4. Land area requirement
5. Hydraulic loading rate
6. Infiltration system design and
7. Groundwater mounding

The RI process is entirely dependent on soil and hydrogeological characteristics at a particular site.

1. The soil must have sufficient hydraulic capacity to allow the wastewater to infiltrate and then percolate and move either to groundwater or underdrains.
2. The fine top textured soil must be removed from site to utilize the underlying coarse soils as the basin bottom and percolation media.
3. The top 5 to 10 ft of soil beneath the basin must be unsaturated at start of flooding to allow the expected treatment to occur.
4. The subsurface conditions should ensure that the percolate can flow away from the site at expected rates.
5. An underdrain can be used to remove renovated water.