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.