By Peter S. Cartwright, PE, CWS-VI
Virtually all of the laws and regulations addressing potable water treatment and waste water discharges are based on from the same fundamental premise—the health of humans using these water supplies must be protected.
Although the contaminants in typical “raw” (untreated) surface and groundwater sources are generally in much lower concentrations than waste water (sea water is the exception), the treatment goal is the same: protect our water resources from contamination. In the case of municipal and industrial water supplies, the raw water represents the “source”, whereas, in waste water applications, these same water sources are the recipient of the discharges. In other words, a river may be the source of raw water supplies as well as the location for treated waste water discharge.
In times past, a distinction was made between water purification and waste water treatment, with system manufacturers producing and marketing to only one of these two classifications. There were even separate trade organizations (who rarely talked to each other); in other words, there was either “water treatment” or “waste water treatment” and “nary the twain shall meet”.
Thanks to a number of factors, the distinction between these two is rapidly becoming blurred. This can be attributed to the following:
- A better understanding and more accurate measurement of contaminants in our source water
- The requirements for more rigorous treatment of water supplies, whether for drinking or non-municipal applications
- Regulations limiting discharges of liquid wastes
- The development and availability of new, more effective treatment technologies
- A growing attitude of ecological responsibility, manifesting itself as water conservation, water reuse and a reluctance to contribute to the contamination of our planet
As a result of all of these factors, many water users are embracing the concept of water recycling: the recovery, treatment (usually required) and reuse of water within a facility. The mindset that embraces the concept of water reuse ensures that the facility has made a conscious decision to reclaim this water and has taken the required steps to:
- determine exactly what contaminants are present, along with their concentrations.
- determine the quality of water required for reuse.
- select and implement the necessary technologies to treat the waste water for reuse.
In comparing raw water treatment with waste water treatment, the approach is quite different. For the former, there has been little motivation to try to recover every drop of water, whereas in waste water treatment, the regulations are such that the waste water generator has strong incentive to recover as much water as possible by concentrating the contaminants a very small volume. These latter highly concentrated streams present a challenge to system designers and require thorough testing to determine the system design parameters, select the optimum technologies and identify pre- and post-treatment requirements.
In reality, the water reuse goal is almost identical to waste water treatment: to recover as much of the water intended to be discharged as possible.
To properly address any water/waste water reuse operation, it is essential to understand the nature of water-borne contaminants. For normal raw water supplies, it is possible to categorize these contaminants into four classes as defined in Table 1.
For waste water streams, the contaminant possibilities are almost endless resulting from largely indiscriminant discharge of manmade contaminants (e.g., machining oils, industrial chemicals, pharmaceuticals, etc.).
Certainly, there is no shortage of treatment technologies for removal of these contaminants. The requirement for system designers is to be knowledgeable enough with the plethora of choices to make an intelligent decision regarding which to select for additional investigation and possibly further testing for their particular waste water stream.
Fortunately, most of the technologies now used for raw water treatment are also excellent candidates for waste water reuse applications. A general description of these follows:
This technology is primarily utilized to remove suspended solids from any water supply. These can be dirt, silt or other particulate material that may interfere with the intended use of the water or a downstream treatment technology. Filtration technologies include:
Bed filters—a tank containing granular media such as sand, anthracite, garnet, etc. which capture suspended solids and retain them inside the bed until it is taken offline and backwashed. Bed filters are typically capable of removing suspended solids down to 10-20 microns in size.
Cartridge filters—operating in the same way as bed filters, cartridge filters are replaceable “inserts,” usually cylindrical in configuration, that are inserted into housings, and are typically replaced when they have captured so much suspended solids that the pressure drop across the housing becomes unacceptable (usually 10 psig). Offered in many different designs and micron removal ratings (down into the submicron range), they provide an excellent array of choices to the knowledgeable design engineer.
Bag filters—these are similar to cartridge filters except that the medium is fabricated into a bag through which the water flows. Although not available with a micron rating as small as cartridge filters, bag filters are generally “tighter” than bed filters.
The four major crossflow membrane technologies in use today are microfiltration, ultrafiltration, nanofiltration and reverse osmosis, which utilize pressure as the driving force. These technologies behave differently than filters in that (with some exceptions) the feed stream is pumped at a high flow rate across the surface of the filter media (membrane), with a portion of this stream forced through the membrane to effect separation of the contaminants, producing a purified stream (permeate) with the concentrated contaminant remaining in the other stream (concentrate) which exits the membrane element on a continuous basis.
Another crossflow membrane technology, electrodialysis, utilizes electrodes and charged membranes to effect separation, rather than pressure. The pressure-driven crossflow filtration technologies are described as follows:
Microfiltration (MF)—this technology is primarily used to remove submicron suspended materials from water supplies. The size range is from approximately 0.01 to 1 micron (100 to 10,000 angstroms). By definition, microfiltration does not remove dissolved materials.
Ultrafiltration (UF)—This membrane process removes dissolved non-ionic solute, typically organic materials (macromolecules). Ultrafiltration membranes are usually rated by “molecular weight cut-off” (MWCO), the maximum molecular weight of the dissolved organic compound that will pass through the membrane into the permeate stream. Ultrafiltration pore sizes are usually smaller than 0.01 micron (100 angstroms) in size.
Nanofiltration (NF)—This technology can be considered “loose” reverse osmosis. It rejects dissolved ionic contaminants but to a lesser degree than RO. NF membranes reject multivalent salts to a higher degree than monovalent salts (for example, 99 percent vs. 20 percent). These membranes have molecular weight cut-offs for non-ionic solute below 1,000 Daltons.
Reverse Osmosis (RO)—Reverse Osmosis produces the highest quality permeate of any pressure driven membrane technology. Certain polymers will reject over 99 percent of all ionic solids, and have molecular weight cut-offs in the range of 50 to 100 Daltons.
Probably the most technically challenging issue in waste water reuse treatment is the effect of the high contaminant concentrations on membrane system design. Highly concentrated suspended solids will increase the propensity of fouling—the bane of all membrane system operations. Certain slightly soluble salts, such as the carbonate, sulfate and phosphate salts of calcium, may exceed their solubility limits and precipitate out.
The degree to which contaminants are concentrated can be defined by the formula in Figure 1.
As stated above, the range of possible contaminants found in a waste water stream is virtually endless, from organic (including oils, greases and other immiscible compounds) to heavy metals to suspended materials such as glass, plastic and metal—contaminants in the waste water from various manufacturing operations. One of the classes of the above contaminants whose treatment warrants special mention is oily waste, discussed below.
Oily waste treatment
Oily waste contaminants are usually the result of a manufacturing or oil drilling operation, and the selection of the appropriate treatment technology is based on the form of the oily waste:
Free oil—forming a discrete layer on top of the water stream, free oil can be removed by skimming or decanting. Because of the immiscibility of oil and water, free oil is readily removed from the water supply.
Emulsions—oil can be emulsified in water by either using special chemicals (chemical emulsion) or high-speed agitation (mechanical emulsion). If the emulsion is stable, it behaves like a suspended solid and can be removed with microfiltration. Otherwise, the emulsion can be broken by acidification and either the free oil is brought to the surface with dissolved air floatation, or a coagulant such as alum added to absorb the oil and form an insoluble sludge. Oil-water coalescers, devices that mechanically break emulsions, are also widely used.
Chemicals—In all of the areas of water treatment, there is a vast array of chemicals which serve as coagulants, flocculants, dispersants, antiscalants and adsorbents, also used in the treatment processes.
Ultrapure water production
In those applications where water quality must be extremely high, called ultrapure water, there are a number of “polishing” technologies available. These include “deionization” using ion exchange resins to adsorb trace concentrations of salts, electrodeionization (EDI), utilizing a combination of electrodialysis and ion exchange resins to produce ultrapure water on a continuous basis, and even distillation in certain applications.
On the other end of the spectrum, if the contaminant stream can be concentrated enough to produce an insoluble sludge, it can be further de-watered through the use of chemicals, a filter press and/or sludge dryer to facilitate disposal.
There is little doubt that today, the only distinction between water and waste water reuse treatment is the source. The treatment technologies are now virtually identical, and the previous barriers to reclaiming waste water for reuse, even to meet the most demanding water quality requirement, have largely disappeared.
About the author
Peter Cartwright, president of Cartwright Consulting Co. is a registered professional engineer in several states. In the water treatment industry since 1974, he has published more than 100 papers and articles on related issues. Cartwright has been chairman of several WQA committees and task forces and received the organization’s Award of Merit. He has been a member of the WC&P Technical Review Committee since 1996. Cartwright can be reached at [email protected] or visit his website, www.cartwright-consulting.com