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.

Vermiculture

Vermiculture

Vermiculture is the culture of earthworms. The goal of vermiculture is to continually increase the number of worms in order to obtain a sustainable harvest. Vermicomposting is the process of converting organic material into a humus-like material known as vermicompost. Production of vermicompost requires maximum worm population density all the time. However, to produce more worms, the population density should be low enough that reproduction rates are optimised. The worm Eisenia fetida commonly known as 'compost worm', 'manure worm', 'red worm' or 'red wiggler' is extremely tough and adaptable can be found wherever piles of manure have been left to age for a few months. The compost worm has a capacity for very rapid reproduction.

Advantages

1. Vermicompost is superior to conventionally produced compost
2. Worms can be used on farms as high quality animal feed
3. Vermicomposting and vermiculture are potential sources of supplemental income to farmers

Disadvantages

1. Rapid production of vermicompost requires more labour
2. It requires more surface area as worms being surface feeders do not operate in material more than one meter deep
3. Vermicomposting is more vulnerable to environmental pressures like freezing and drought
4. Start-up resources in the form of initial investment, time and labour are required

Compost worms need:

1. A conducive environment to thrive called 'bedding'
2. Food
3. Adequate moisture (>50% water by weight)
4. Adequate aeration
5. Protection from temperature extremes

'Bedding' provides worms with a stable habitat. It should have the following essential characteristics:

1. High absorbency
2. Good bulking potential
3. High Carbon:Nitrogen (C:N) ratio

The bedding material may be made-up of 'peat moss', 'horse manure', 'Newspaper' or 'paper mill sludge'. Selection of bedding material is very important to successful vermiculture. Manures are the most commonly used feed stock. The bedding must hold sufficient moisture for the earthworms to thrive. Worms cannot survive in anaerobic conditions. Earthworms thrive in temperatures in the range of 20s(C). Such temperature ranges stimulate reproduction. However, they die in temperatures exceeding 35(C). Compost worms will redistribute themselves according to temperature gradient.

Worms can survive in the pH range of 5 to 9. Worms are very sensitive to salts, preferring salt contents less than 0.5%. Few toxic components to earthworms are:

1. De-worming medicine in manure
2. Detergent cleansers, industrial chemicals and pesticides
3. Tannins

Earthworms in ideal conditions reproduce quickly. Worm populations double every 60 to 90 days. Ideal conditions being:

1. Adequate food
2. Well aerated bedding with moisture content between 70 and 90%
3. Maintaining temperature between 15 to 30 C
4. Initial stocking densities more than 2.5 kg/m2 but less than 5 kg/m2

Stocking density refers to the initial weight of worm biomass per unit area of bedding. Starting with a population density less than stocking density will delay the onset of rapid reproduction. Population density of worms greater than stocking density results in low reproduction as there is greater competition for food and space. The most common densities for vermicomposting are between 5 and 10 kg/m2. Worm growers tend to stock at 5 kg/m2 and tend to split beds when the density has doubled. Following these guidelines, growers can expect doubling of biomass in 60 days. Theoretically a stock of 10 kg of worms can become 640 kg in after one year and 40 tonnes after two years. The barriers in achieving optimum rates of reproduction are:

1. lack of knowledge and experience
2. lack of dedicated resources
3. lack of preparation for winter

Rule of thumb is that one ton of input results in one cubic yard of compost.

The most common pests and diseases that earthworms are at a risk of are:

1. Moles
2. Birds
3. Centipedes
4. Ants
5. Mites
6. protein poisoning

The three basic types of vermicomposting systems are:

1. windrows
2. beds or bins
3. flow-through reactors

• Vermiculture focuses on production of worms rather than vermicompost.
• Vermicompost can be used as a method for destroying pathogens.
• Vermicompost spread on land does not cause contamination of ground or surface water.
• Vermicompost binds nutrients well thereby preventing nutrient run-off from agricultural land and ultimately preventing eutrophication of surface waters.
• There is potential for using compost worms in natural filtration systems.
• One of the principal benefits of vermicomposting is 'carbon sequestration'.
• Vermicomposting also addresses the issue of worldwide depletion of carbon in soils. By consistent application of compost or vermicompost an increased level of carbon in soil has been seen.
• Worms aerate the matter as they move through it resulting in fewer anaerobic areas and reduced methane emission.
• One unit of vermicompost is as effective as five to seven times of fertilizer in promoting plant growth and yield. Vermicompost is more efficient at retaining nitrogen.
• Earthworms have a very important role in counteracting 'the loss of biodiversity'.
• Vermicompost has a high potential value monetarily.
• Vermicomposting and vermiculture are environmentally beneficial processes that have great potential as components of sustainable agriculture.

Exercise problems

EXERCISE PROBLEMS

Design an aerated lagoon given the following data:

• Wastewater flow = 12,400 cu.m/day
• BOD5 =  300 mg/L
• Population = 70,000 people
• Detention period = 3 days
• k' = 0.015 d-1 at 20℃
• Y=0.5
• Kd = 0.07 d-1 [BOD5 basis]
• Use ideal  complete-mixing model
• Lagoon is proposed to be followed by another treatment unit

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Design a flow through aerated lagoon to treat a wastewater flow of 3800 cu.m per day. The treated liquid is to be held in a settling basin with a 2 day retention time before discharge. Total biological solids produced are equal to computed volatile solids divided by 0.8. Assume: 

• Influent suspended solids (non biologically degraded) = 200 mg/L; 
• Influent soluble BOD5 = 200 mg/L; 
• Effluent suspended solids after settling = 20 mg/L; 
• Kinetic coefficients:
•  Y = 0.65; 
• Ks = 100 mg/L; 
• 𝞵max = 6.0 d-1; 
• Kd = 6.07 d-1; 

• Lagoon depth = 3m; 
• Design mean cell retention time = 4 days

1. On the basis of mean cell retention time, determine the surface area of the lagoon. 
2. Estimate the soluble effluent BOD5 measured at the lagoon outlet
3. Estimate the concentration of biological solids produced
4. Estimate the suspended solids in the lagoon effluent before settling

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Design a facultative aerated lagoon  to serve 40,000 people given the following data:

• Sewage flow = 180 LPCD
• Raw BOD5 = 50 g/person/day
• BOD5 should not exceed 40 mg/L in winter
• Average temperature of air in winter = 18℃
• Average temperature of air in summer = 37℃
• KL = 0.7 per day and Kd = 1.2 per day at 20℃
• Coliforms in sewage - 10^7 per 100 ml

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Design a flow-through type aerated lagoon without recycling for the following data:

• Volume of wastewater to be treated = 3.0 MLD 
• Influent suspended solids (non-biodegradeable) = 150 mg/L
• Effluent suspended solids = 50 mg/L
• Influent soluble BOD5 = 150 mg/L
• Effluent soluble BOD5 = 150 mg/L
• Summer air temperature = 35℃
• Winter air temperature = 25℃
• Wastewater temperature = 27℃
• Mean cell residence time (ϴc) = 3 days
• Water depth in the lagoon = 3.5m
• Elevation of plant = 600m

Assumed data:

• Proportionality constant (f) = 0.5
• Oxygen concentration in aerated lagoon = 1.5 mg/L
• First order soluble BOD5 rate constant, Kbase e = 2.5 d-1 at 20℃

• Mixed Liquor Volatile Suspended Solids, X = 0.8 * MLSS (MLSS = Mixed Liquor Suspended Solids)

• Aeration constants α = 0.85 and β = 1.0

Temperature coefficient = 1.06

Kinetic coefficients:

                                Y = 0.6

                                K = 6.0 d-1

                                Ks = 90 mg/L

                                Kd = 0.06 d-1

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Determine the aerated lagoon size and power requirements to serve 50,000 people assuming sewage generation rate = 250 LPCD, influent BOD5 = 300 mg/L and effluent BOD = 20 mg/L.
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Design a facultative pond in a waste stabilization pond to treat 3.5MLD of wastewater which has a design loading of 440 kg BOD/ha-d. The design temperature is 30C.
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References

References

1. Wastewater Engineering - treatment, disposal and reuse - Metcalf and Eddy
2. Wastewater treatment for pollution control - Soli J Arceivala
3. Environmental Engineering - Kiely Gerard
4. Environmental Engineering: Sewage disposal and airpollution engineering - S.K.Garg
5. Environmental Engineering - Keiley
6. Environmental Engineering- A design approach - Sincero & Sincero
7. Unit Operations and Processes in Environmental Engineering                  - Tom D. Reynolds
8. Introduction to Environmental Engineering - P. Aarne Vesilind

Design considerations for aerated lagoons

DESIGN CONSIDERATIONS

• The following factors  should be kept in mind when designing an aerated lagoon

1. The desired quality of effluent is based on concentration of solids to be maintained in the effluent
2. BOD5 or COD removal gives the efficiency of substrate conversion
3. Quantity of oxygen to be supplied
4. Power needed for mixing and supply of oxygen
5. If the temperature variations between summer and winter are significant, the effect of temperature should be considered.
6. The design criteria required for designing an aeration system are:
1. Detention time: Suspended growth aerated lagoons are designed on the basis of Hydraulic Retention Time (HRT) and Mean.Cell Residence Time (mCRT) or Solids Retention Time,𝜃c (SRT). Typical design value of SRT or HRT for suspended growth aerated lagoon is ~ 3 to 6 days while for facultative aerated lagoon is ~ 4 to 10 days.
2. Oxygen requirement: The oxygen required for oxidising organic solids varies from 0.7 to 1.4 kg of oxygen per kg of BOD5 removed. Oxygenation capacity of aerators varies from 1.85 to 2.0 kg O2 per kW of power delivered under standard conditions. Oxygenation capacity of aerator at field conditions is given by the equation:                                               N = [Ns (Cs - CL) * 1.024^(T-20a)] / 9.2
3. Power requirement: Power required for mixing the contents of the aerated basin varies from 0.8 to 1.0 kW/1000 cu.m of basin volume. The power required to keep bio-solids in suspension is 1.5 to 1.75 kW/1000 cu.m and the power required to keep ALL solids under suspension is 15 to 18 kW/1000 kW/1000 cu.m
7. The assumptions for the design of an aeration system are:
1. Oxygen content = 2302%
2. Diffuser efficiency = 30 to 50% (or as per manufacturer specifications) 
3. Field Oxygen Transfer Efficiency = 50% (or as per manufacturer specifications) 
4. Air weight (density) = 1.201 kg/cu.m for aerobic reactors
8. Criteria adopted in design of aerated lagoons:
1. mCRT = 3 to 6 days without recycle for domestic wastewaters and 10 to 30 days with recycle for domestic wastewater
2. Oxygen requirement = 0.7 to 1.4 * BOD5 removed, kg/d
3. Solids concentration in lagoon  = 30 to 300 mg/L for aerobic flow through type                                                              = 30 - 150 mg/L for facultative type and                                                                        = 4000 to 5000 mg/L for extended aeration type
4. Hydraulic detention time = 2 to 10 days for aerobic flow through type  

                                                                   = 3 to 20 days for for facultative type and                              
                                                                   = 0.2 to 7 days for extended aeration type
                           5. Depth of lagoon = 2 to 5m
                           6. Power required for oxygen supply = 1 to 8 HP/1000 cu.m of basin volume
                           7. Oxygen transfer capacity of surface aerators = 1.85 to 2.0 kg O2/kW h at standard     
                                                                                                      conditions
                9. Lagoon surface area is assumed rectangular in the ratio 1.75 - 1.95:1 (Length:Width)

               10.Dispersion number (D/UL) = 0.2 to 1.0 for rectangular or long lagoons and 2 to 4 and above 
                                                                 for squarish lagoons
               11. BOD5 removal efficiency = k * t  where k = 0.776
               12. %BOD removal efficiency is determined using the graph showing wehner-wilhelm equationc   
                      for substrate removal efficiency based on dispersed flow model
               13. Sludge accumulated is determined using a cleaning interval of 3 years
               14. Dispersion number (D/UL) generally lies between 0.1 and 0.4
                       if (D/UL) ~ 0.2 or less the reactor is a plug flow reactor
                       if (D/UL) ~ 3.0 to 4.0 the reactor is well mixed or is approaching complete mixing
 USEFUL FORMULAE:

Power = Work/Time and the units of power is W (watt)

Watt = Joules/second

Horse Power (HP) = 750 W

Power = Force * Displacement / Time = Force * Velocity

V = Q * t

Hydraulic loading or surface loading rate or Overflow rate = flow (m3/d) / surface area (m2)

Weir overflow rate = flow rate / total weir length

Organic loading = Applied kg of BOD per day / Volume of tank

Mean Cell Residence Time (mCRT) or Solids Retention Time (SRT) = 

Biomass in reactor / Biomas removed from reactor

N = [Ns (Cs - CL) * 1.024^(T-20a)] / 9.2

N = Oxygen transferred under field conditions in kg O2/h

Ns = Oxygen transfer capacity under standard conditions in kg O2/h

Cs = Dissolved Oxygen saturation value for sewage under operating temperature

CL = Operating Dissolved Oxygen level in aeration tank 1 to 2 mg/l

T = Temperature in oC

a = Correction factor for oxygen transfer for sewage (0.8 to 0.85)

IMPORTANT QUESTIONS

EFFLUENT DISPOSAL

 EFFLUENT DISPOSAL

• Receiving water standards 

• Disposal of effluent in lakes and rivers

• Mathematics of mass transport

• Diffusion - advection

 

• Hydraulic models of physical systems (Continuous flow stirred tank reactor model & plug flow reactor model)

• Disposal of effluent into the ocean

• Outfall design