In the U.S., utilities are required to provide source water treatment prior to distributing potable water to their customers. Production of potable water from most source waters (groundwater, seawater and surface water) typically requires several steps of treatment to meet drinking water standards. For complex water treatment, the three categories of treatment are: pretreatment, core treatment and post-treatment. The core treatment is generally the step that results in complying with the regulations. Pretreatment allows the core treatment to be more effective in terms of production and will, therefore, result in lower operation and maintenance (O&M) costs. The post-treatment step is used to improve the quality and stability of the finished water and to provide a disinfectant residual.
Why pretreat?
Pretreatment during the water treatment process is critical to sustain productivity and minimize O&M costs. Before implementing the construction of a water plant, careful consideration must be taken to select a pretreatment system. No system provides a one-size-fits-all effectiveness for all facilities. The system that works for one utility may not work for another with similar water quality.
A detailed evaluation of the source water quality should be performed to determine which pretreatment and treatment are best-suited to meet the finished water quality goals. Water chemistry should be evaluated to understand the pretreatment impacts on the partially treated and/or finished water quality. For example, chlorine could be used to control taste and odor. However, chlorine contacting source water organics could result in the formation of unwanted disinfection byproducts.
Pretreatment options
The primary goal of pretreatment is to remove any substances from the source water that may damage or be detrimental to the core treatment. Pretreatment can also be used to enhance the effectiveness of the core treatment.
Pretreatment options include advanced treatment such as the following:
- Microfiltration/ultrafiltration (MF/UF)
- Coagulation/sedimentation/filtration (CSF)
- Chemical treatment
Seeing MF/UF or CSF as a pretreatment system may be surprising since many plants use these processes as core treatment. However, pretreating with these technologies may be required before a membrane treatment system for source water with high salt content (seawater).
Chemical treatment has different applications depending on the type of chemicals and the raw water quality. The reason for chemical pretreatment is to optimize the treatment operation downstream of the chemical injection and to assist in removing unwanted contaminants.
For example, an antiscalant and/or sulfuric acid are added to prevent the precipitation of salts and some metals in a membrane system. Sulfuric acid can also be used prior to coagulation to ensure that the pH is optimum for CSF treatment.
Potassium permanganate is commonly used to control zebra mussel growth in a reservoir, intake system or raw water pipeline prior to any surface water treatment. Another example is the addition of ozone prior to a sand/anthracite filter to enhance the removal efficiency for most surface water sources.
Pretreatment comes in many shapes, and only a careful analysis will determine the best fit for supporting core treatment systems.
Pretreatment case study
The following example addresses the selection of a pretreatment system prior to a membrane process as the core treatment. Membrane pretreatment system designs can vary considerably and directly affect plant production sustainability. Inadequate pretreatment will result in poor performance of the pretreatment system and the core, reverse osmosis (RO) membrane system. Poor performance results in high O&M costs.
For surface water desalination, pretreatment represents the most critical treatment step because of the higher occurrence of suspended solids and organics levels associated with this source water. The design team should consider different filtration technologies — such as one- or two-stage media filtration — or membrane filtration (MF or UF). If suspended-solids concentrations are low (with the use of offshore water or beach well groundwater), direct filtration may be a feasible pretreatment option.
Key design criteria for pretreatment systems include the loading rate for a media filtration system or the membrane surface flux rate for an MF/UF system. The loading and flux rates have direct impacts on the backwash, cleaning and media replacement frequencies, directly impacting the operation of the core treatment components.
Pretreatment designs should also address potential biological growth issues. Some options to control or minimize biological growth and fouling include ultraviolet deactivation, an advanced oxidation process, injection of a biocide and intermittent cleaning with a disinfectant.
To fully identify the most efficient pretreatment process, including the pretreatment in a pilot study is imperative. A pilot study can help confirm and validate the effectiveness of the selected pretreatment process. The study’s results will help identify the most effective processes as well as the potential capital and O&M costs to evaluate the most cost-effective option. This testing and evaluation process provides the most reliable information for the design, while giving utilities a high level of confidence prior to committing to the construction.
Unlike surface water desalination, the pretreatment requirements for groundwater sources can be minimal. Pretreatment typically consist of cartridge filtration and the addition of a scale inhibitor coupled with an acid to control salt precipitation on RO membranes.
Careful, upfront analysis saves money
Pretreatment technologies come in all shapes and sizes. From chemical to filtration to membrane separation, each system’s core treatment must be carefully analyzed to determine the best option for pretreatment. Pilot studies are extremely useful tools to confirm and validate optimal core and pretreatment system performance. To be fiscally responsible in engineering, capital and operational expenditures, the full life cycle of each treatment step should be evaluated and potentially piloted. Learning the operational challenges during a pilot study is much easier and cheaper to manage than during full-scale construction and operation. The cost implications will be magnified by a minimum factor of 100 after startup.
Christophe Robert (process engineer) and Lance Littrell (client service manager) are engineers with Reiss Engineering, a civil and environmental engineering firm specializing in the design and construction of water treatment, wastewater and reuse water facilities, with experience in master planning and permitting. They have more than 30 years of combined experience as drinking water treatment processes and facility design engineers. Their expertise centers on water quality and advanced treatment — including ultrafiltration and RO for the treatment of groundwater, surface water and seawater for small and large utilities. Robert may be reached at [email protected] or 407-679-5358. Littrell may be reached at [email protected] or 407-679-5358.
