Methods for Dealing With Low-quality Groundwater
Written by AOS Treatment Solutions on December 19, 2016
Many of those with water-treatment responsibilities find that regulatory demands increase as overall groundwater quality decreases. The high-quality groundwater that is easily treated and delivered for drinking and other purposes often is in short supply.
Reasons for Low-quality Groundwater
There are a number of reasons for the decreased quality of local groundwater.
Chlorides and Sulfates
Sometimes, alluvial aquifers near streams and rivers are recharged with surface water loaded with chlorides and sulfates. Runoff from agricultural sources, residential areas and even golf courses may contain high chloride and sulfate levels.
Bacterial and Viral Pathogens
Groundwater may also be compromised by bacterial and viral pathogens found in fecal matter which reaches groundwater sources through large cracks in the ground, simple soil penetration, failed septic systems and leaking sewer lines.
Meeting Water Quality Standards
The Texas Commission on Environmental Quality promotes water quality through a number of mechanisms, including enforcement of the Ground Water Rule (GWR).
Also, water-treatment facilities must adhere to EPA standards pertaining to Maximum Contaminant Levels (MCLs) established for chemical compounds with adverse health consequences. Water aesthetics are addressed through Secondary MCLs (SMCLs). In general, problems with how municipal water supplies look, smell or taste quickly produce negative feedback from the public.
There are a variety of ways that water-treatment facilities can cope with excessive MCL and SMCL levels.
Aeration is important when treatable water contains high concentrations of carbon dioxide. It is also used to strip out hydrogen sulfide and ammonia, and to oxidize manganese and iron, as well as to remove radon and certain gaseous VOCs.
Precipitative softening often follows aeration or pre-sedimentation. It reduces hardness and improves clarification. The process often includes these steps: rapid mixing, flocculation, sedimentation, recarbonation and final filtration.
Ion Exchange (IX)
This water-treatment method is used to remove magnesium and calcium ions. However, according to the EPA, this process is being used more often for removing contaminants like sulfate, nitrate, fluoride and arsenic. In general, it is capable of removing most MCLs.
Electrodialysis Reversal (EDR)
EDR is a water-treatment solution which is capable of removing numerous problem compounds. It employs an electrodialysis stack to remove sulfate, nitrate and fluoride ions. This desalination membrane process has been in use since the 1960s.
Reverse Osmosis (RO)
RO can effectively remove MCLs and SMCLs. It is often a worthwhile option because RO systems are modular in nature and easily installed. The system is scalable, so it is readily expanded if and when treatment capacity must increase.
Contaminant Disposal Options
The treatment of low-quality groundwater generates waste products which must be disposed of in an environmentally and legally acceptable way. Various options include:
Aeration releases dissolved gases into the atmosphere in adherence to dispersion calculations which account for the concentration and direction of the plume.
Discharge into Waterways
Return of concentrated waste into local waterways is possible only after the required NPDES permit is acquired. IX treatment generates a concentrated brine which includes both contaminants and the chemical used to recharge the exchange resin. EDR and RO also produce concentrated contaminants which require disposal.
The sludge generated by the lime-softening process is recycled in cement kilns, at coal-fired power plants or to alter soil composition.
Deep Well Injection
Deep well injection is a permanent solution to contaminant disposal. It is preferred over the use of evaporative ponds because of possible pond leakage and poisoning of landing waterfowl. Deep well injection is appropriate when the injection zone is fully sealed off from water-bearing zones.
Sophisticated treatment methods that were once too complicated or expensive to deploy are now viable options for addressing low-quality groundwater-treatment requirements. Treatment options and waste product disposal must both be considered when determining the most cost-effective solution for each special set of circumstances.