Wastewater treatment technologies can also be designed to provide low cost with additional benefits from the reuse of water. These systems may be classified into three basic systems as shown under:

1. Mechanical treatment systems

2. Aquatic systems

3. Terrestrial systems

Mechanical treatment systems use natural processes within a constructed environment. Such systems are usually applicable where suitable lands are unavailable for the implementation of natural system technologies. However, aquatic systems are in the form of lagoons; facultative, aerated, and hydrograph controlled release (HCR) lagoons. These lagoon-based treatment systems can be provided with additional pre or post-treatments using constructed wetlands, aqua cultural production systems, and sand filtration wherever required. Terrestrial systems use the nutrients available in wastewaters which facilitate plant growth, soil adsorption, and converting biologically available nutrients into less-available forms of biomass. Such a system is used methane gas production, alcohol production, cattle feed supplements etc.

Waste water treatment methods may also be classified under following

a. Primary treatment

b. Secondary treatment

c. Tertiary treatment

Wastewater treatment consists of physical, chemical and biological methods basically used to remove the contaminants from wastewater. In individual wastewater treatment, procedures/ techniques are combined into variety of systems in order to achieve different levels of contaminant removal. These treatments are classified as Primary, Secondary and Tertiary waste water treatment.

Primary treatment systems are used to remove suspended solids, oil and grease, floating materials, mixing of coagulants and coagulant aids, and removal of pollutants through well designed settling systems. These primary systems are usually in the form of physico- chemical treatment where inorganic impurities are removed which helps in reducing the pollution load in secondary treatment.

a. Screening

b. Grit chamber

c. Oil and grease trap

d. Equalization and neutralization

e. Coagulation and flocculation

f. Sedimentation tank (settling tanks or clarifier)

g. Flash mixture

Secondary treatment is employed after primary treatment to remove organic pollutants present in the wastewater. Such a system has different residence time for different unit processes. The unit processes and operations used in secondary treatment are listed below:

a. Activated sludge process

b. Trickling filters

c. Lagoons

d. Oxidation ponds

e. Anaerobic digestion

Tertiary treatment is employed as an advance treatment system to remove remaining left over organic and inorganic impurities in the waste water after primary and secondary treatment. The unit operation and processes used in tertiary treatment are listed as under.

a. Chlorine (or other disinfecting compounds, or occasionally ozone or ultraviolet light)

b. Reverse Osmosis

c. Filtration

d. Desalination

e. Colloidal removal

Various treatment technologies of wastewater in the form of Activated sludge treatment 12, Aerated lagoons, Oxidation ponds 13, Trickling filter 14, Rotating biological contactors 1516, Sequencing batch reactor 1718. Anaerobic treatment 19, etc. were tried for treating dairy wastewater. 20 Published a review on water utilisation, energy utilisation and wastewater management in the dairy industry. 21 Reviewed and investigated environmental impact of dairy effluents and their effective treatment using biological wastewater treatment technologies. 22 showed mathematically that a two-stage aerated lagooning system with an aerobic lagoon as the first stage and a facultative lagoon as the second stage will require less total detention time than it would if a single aerobic lagoon or a single facultative lagoon were used (or if two or more facultative lagoons were used in series). 23 performed an aeration experiment in a dairy lagoon with two commercial aerators for 1 month. Liquid concentrations of ammonia, total nitrogen, nitrite and nitrate were monitored before, during and after the experiment and atmospheric ammonia was measured downwind of the lagoon using a short-path differential optical absorption spectroscopy (DOAS) instrument with 1 ppbv sensitivity. Combined photosynthesis and mechanical aeration for nitrification in dairy waste stabilisation ponds was studied by 24. 11 reviewed the performance and design criteria of constructed wetlands sand filters for the treatment of dairy wastewater. 25 evaluated dairy wastewater for biological hydrogen (H₂) production in conjugation with wastewater treatment in a suspended growth sequencing batch reactor (AnSBR) employing sequentially pre-treated (heat-shock (100 ◦C, 2 h) and acid (pH 3.0, 24 h)) mixed consortia. 26 studied the treatment and stabilisation of dairy wastewater using limited aeration treatments. A single chamber microbial fuel cell with spiral anode for dairy wastewater treatment was developed by 6. 10 harnessed the redox gradients in facultative lagoons using a lagoon microbial fuel cell (LMFC) to enhance autonomously the delivery of oxygen to the lagoon through aeration and mixing by operating an air pump. 9 determined behaviours of various parameters of dairy wastewater and evaluated the performance of effluent treatment plant. 27 evaluated aerated lagoon using life cycle approach. Aim of present study was to design aerated lagoon for dairy wastewater treatment.

Aerated lagoons are one of the well-known treatment system required for the treatment of dairy wastewater and it works as a proficient and easy approach for removal of the organic and inorganic loading in the dairy effluents 28. The lagoons can be operated both in aerobic and anaerobic condition depending on the type of wastewater released from the industry

The aerated lagoons are used frequently for the treatment of industrial wastes because of their simple operations, removal efficiencies and less land requirements. Combination of lagoons can be used where lagoon water will recirculate as flushing water in a confinement facility and further treatment is done. Aerators are generally placed on the lagoon surface that provide enough oxygen for aerobic oxidation and also allow a sludge layer to form at the bottom of partially mixed lagoons29.

An anaerobic lagoon followed by a naturally or mechanically aerated lagoon will provide flushing water that does not have disagreeable odours and more attractive treated manure for land disposal by irrigation. This method ensures a subsequent “self-digestive process” of the biomass by the optimal use of the biological reaction to degrade the pollutant 30. It is a good treatment plant that runs excellently, give you a lot of extra time to do other jobs. This system is a time saver.

An aerobic lagoon is one in which the mixing level created by the aeration equipment keeps the solids in suspension. A facultative lagoon is one in which the mixing level is low enough to allow solids to settle but high enough to distribute the dissolved oxygen (do) throughout the lagoon. The design, functioning and efficacy of aerated lagoons depend on temperature, type of microorganisms and their nutrient uptake rate 31.

The wastewater from 5 industries located in Rajasthan were taken and analyzed. The characteristics of wastewater vary substantially partly due to utilization of production capacity at that point of time and partly on account of the size of the industries and practices adopted. The salient characteristics are shown in Table 1 below and schematic layout of treatment system is also shown in Figure 1, Figure 2, Figure 3.

The design of the treatment plant is based on the average values as given here under:

BOD in mg/l = 1000

Volume in m^3/day = 4000

Assuming designed rate of flow to be 1.5 times the average flow

Hence designed volume to be handled is = 1.5*4000 = 6000 m³/day or 0.07 m³/sec

Ideal velocity flow through velocity is assumed as 0.6 m/s

Area required to accommodate flow = 0.07/0.6 = 0.117 m²

Assuming width of screen chamber =0.5 meter

Depth of flow = 0.117/0.5 = 0.234 meters or 23.4 cms

Using 12 mm rectangular bars at 50 mm centre to centre distance

Clear opening = 38 mm

End clearance =38 mm

Let there be n bars

Total width of opening = ( n + 1 )*38 = 500

n = (500 - 38)/ 38 = Say 13 nos

Total width of the screen chamber = 50 + 13*1.2 = 65.6, say 70 cms

While designing primary clarifier, following assumptions are taken

High performance flow rate = 20 m³/m²/day

Clear water depth = 3 meters

Loading rate = 20 = 6000/ (π/4)*D²

D = 19.54 meters, say 20 meters

Detention time = Volume of clarifier/ wastewater quantity

Volume of clarifier = 3* (π/4)*D²

= 3*3.14*20²/4

=942 m³

Wastewater quantity = 6000 m³/day

So detention time = 942/6000 = 0.157 days = 3.768 hours

It is expected that primary clarifier would remove 30 percent of the influent BOD and also 80 percent of suspended solids.

Hence BOD after primary clarifier = 0.7*1000 = 700 mg/l

The design data for aerated lagoon after the primary clarifier is as under:

BOD influent = Li = 700mg/l

System rate constant = K = 0.12/days

Oxygen requirement for 90 percent removal of BOD = 1.4kg/kg BOD applied

Oxygen capacity of surface aerators = 1.36kg O₂/H.P/hr

Liquid depth = 3 meters

Free board = 0.3 meters

Shape = Rectangular

Length: Breadth = 2 : 1

Side slope = 1 vertical: 1 horizontal

Effluent BOD after lagoon treatment =30 mg/l

Design calculations of Aerated Lagoon:

Detention time, t = log (Li/Le)/ K = log (700/30)/0.12 = 11.40 days, say 12 days

Hence volume of lagoon V = Q *t = 6000*12 = 72000 m₃

Providing 2 lagoons of equal size

Size of each lagoon V = 72000/2 = 36000 m3

V = 36000 = {2a*a) + (2a-6) (a-6)*3}/(2)

a = 98.2 or say 99 meters

Hence size of lagoon = 99 meters width and 198 meters length

Hence size of lagoon =

Assuming 90 percent BOD reduction

Oxygen requirement = 1.4 kg/kg of BOD applied

Total kg of BOD applied = 6000*1000*700/ 10⁶

= 4200 kg/day

Oxygen required = 1.4*4200 kg O₂/ day = 245 kg/hr

Assuming surface aerators capable of transferring 1.36 kg of O₂/ H.P/ Hour at lagoon condition

Total H.P required = 245/1.36 = 180 H.P

Providing 8 aerators of 22.5 H.P capacities each, each lagoon compartment to have 4 aerators.

Plan and elevation of designed aerated lagoon is shown below in Figure 4.

This zone may be a diked-off portion of the aerated basin

Assuming detention time for quiescent settling = 2 days

Volume of settling basin = 2*6000 = 12000 m³

Providing one settling basin for each lagoon

Volume of each basin = 12000/2 = 6000 m³

Assuming that the width of the basin is equal to lagoon, that is, 99 meters and depth 3 meters

Length of settling basin = 6000/99*3 = 20.2 or say 21 meters

Hence size of settling basin would be 99 meters by 21 meters

Average suspended solids = 1500 mg/l

Assuming 80 percent removal of suspended solids

Suspended solids removed = 1500*0.8 = 1200 mg/l

Quantity of settled solids = 1200*6000*1000/ 10⁶

= 7200 kg/day

Assuming primary sludge contains 4 percent solids by dry weight

Hence the volume of settled sludge = 7200*100/4 = 180000 litres/day

= 180 m³ /day

If the dairy industry is located in rural area and sufficient land is available, the primary sludge can be discharged to sludge lagoon to settle the sludge. The sludge will compact at the bottom. There can be series of such lagoons. When one lagoon is full of sludge, the operation can be shifted to another lagoon. It has been reported that lagoon constructed at some distance from the dairy industry and adjacent to community, have posed aesthetic problems. This problem is proposed to be minimized by developing extensive green belt of selected plant species around lagoon.

The treated wastewater can be applied for raising green belt, green spaces and vegetative cover around the wastewater treatment plant while keeping hydraulic loading concept. Since the treated waste contain adequate nutrients, the vegetative cover will grow fast without using fertilizers. The specific plant species would be selected to grow under local conditions. Such a green infrastructure would not only provide aesthetic atmosphere but also reduce odor, air pollutants, and noise, along with restricting the wastewater to join any water body. However, a comprehensive green infrastructure needs to designed, planned and implemented on a scientific scale.