The Screws Tighten on Wastewater TreatmentBrad Buecker, Process Specialist, Kiewit Engineering and Design Co. | March 29, 2016
The U.S. Environmental Protection Agency (EPA) is expanding efforts to reduce not only leaks of hazardous chemicals at thousands of industrial plants across the country, but also discharge of harmful compounds in wastewater streams from these plants.
The latter may include heavy metals, organic compounds and oils, and non-metal inorganics. Well-known methods and/or emerging technologies may be necessary to remove these pollutants. An overview may prove useful of several technologies for removing problematic wastewater constituents.
Heavy Metals Control
At many industries, heavy metals may be present in plant waste streams. These metals may include cadmium, chromium, copper, lead, nickel, titanium, vanadium, and zinc, among others. In the electric power generating industry, particularly at remaining coal-fired plants, arsenic, mercury, and selenium are of primary concern. Each exhibits a varying degree of toxicity to aquatic organisms and humans, but in most cases wastewater discharge guidelines limit concentrations to low parts-per-billion (ppb) values, or even lower in the case of mercury.
One established process for achieving such low concentrations is precipitation of the metals at high pH in clarifiers, where the metals come out as hydroxides (M-OH).
Lime [Ca(OH)2] softening clarification has been used for decades to precipitate hardness and bicarbonate alkalinity into a sludge that can be converted into a filter cake and disposed. However, as the figure illustrates, minimum solubility for metals occurs at varying pH values. Thus, treatment systems using hydroxide precipitation must be tailored to establish conditions for maximum removal of the metals of concern.
With some metals, hydroxide precipitation may not reduce the concentration to effluent guidelines, or the limit may be so low that special techniques are needed. A case in point is mercury. Consider the power industry where new EPA Effluent Limitation Guidelines for wet scrubber wastewater limit the average mercury concentration to 356 nanograms per liter (ng/L). A number of metals (and mercury most noticeably) react strongly with sulfide (S2-) to form an insoluble precipitate.
For many years, inorganic sulfides were used for metals precipitation, but two issues with inorganic sulfides can be problematic.
First, many inorganic sulfides are toxic and require special handling. Second, and especially in the case of mercury, the reaction of the metal with sulfide is so strong and fast that small particles form, which can be difficult to remove by clarification and/or filtration. Water treatment companies have now developed polymers with active sulfide groups that are applied in clarifiers similar to traditional flocculants. The polymers react with metals and exit with the clarifier sludge just as their normal counterparts do.
These are not all. For years, it has been known that metals will co-precipitate with iron oxides. First, consider basic clarification. Often, the coagulant is iron based, with common coagulants being ferric chloride (FeCl3) and ferrous sulfate (Fe2SO4). Upon addition to the clarifier influent these compounds initially form iron hydroxides, where the basic coagulant purpose is to reduce the negative charge typically found on suspended particles.
Charge neutralization allows the particles to draw more closely together, such that they can then be flocculated. However, the iron hydroxides and oxides will also form a precipitate with metals.
Companies have advanced this chemistry in innovative ways. Two methods that have come to this author’s attention recently are Evoqua’s Pironox process and Veolia’s SeleniumZero technology. These systems use iron oxide particulate circulation in moving-bed reaction vessels to co-precipitate metals. For streams with multiple metals, a series of reaction vessels may be needed for chemistry adjustment, including pH, to achieve the required removal.
Finally, in some cases ion exchange may be an option. Many readers will no doubt think of ion exchange as a common makeup water technology, where the ion exchange resins remove hardness, chloride, sulfate, silica and other dissolved ions to produce demineralized water for feed to high-pressure steam generators or other applications that need high-purity water. The resins remove a variety of ions in a stream, and thus can exhaust quickly when used for wastewater treatment. However, by manipulating the chemistry of the ion exchange support structure and active sites, resins can be manufactured that are ion selective. Arsenic removal is one potential application for selective resins.
Dealing with Oils and Organics
Streams at many chemical process industry (CPI) plants may contain oils or other organics that would cause problems in plant discharge. A number of techniques are available to remove these impurities.
Most obviously, oil skimming can be an effective and inexpensive technique for removing the bulk of any oil in a waste stream. A detailed discussion of skimming is not necessary, other than to say that good engineering practice is needed to install these basic systems. (The author once observed an oil skimmer that was placed well above the elevation of a sump, with submersible pumps that collected water contaminated with oil. Because the oil floated on top of the water in the sump, the skimmer would only see oil when the depth in the sump became great enough for the pumps to move it.)
Even with bulk oil removal by skimming, residual oil may still be problematic with regard to waste stream discharge, or with water recycle to the process for reuse.
Technologies such as dissolved air flotation (DAF), induced air flotation (IAF), and others may be necessary to remove residual oily wastes. One such process is illustrated in the figure.
These systems may be designed with numerous subtleties and modified configurations, but the basic process remains the same. Air is introduced to a flotation vessel or vessels to produce small air bubbles that attach to oily matter and carry it to the vessel surface for subsequent skimming by baffles.
If even slight residual oil remains a problem, treatment of the effluent by filtration through activated carbon is an option. In fact, activated carbon filtration is an effective technique for removal of many organics, including “taste-, odor-, color-, and toxic-promoting species.” 
The author worked for many years with activated carbon systems for power plant makeup water treatment, and these systems will remove many impurities, although regular change-out, perhaps even on a yearly basis is necessary to prevent process contamination due to exhaustion of the media. Anyone considering making use of this technology is advised to discuss the application with activated carbon experts.
Activated carbon may be produced from a variety of raw starting materials, including coconut shells, wood, and coal, and it can be produced in a variety of sizes including powdered form and granular. A typical specification includes the iodine number, which indicates the materials’ adsorptive capacity.
And even activated carbon may not be sufficient for removal of small chain organics, and those that essentially have no charge or dipole moment, for example, small-chain alkanes. Thermal oxidation of the stream may be the most reasonable alternative in these cases.
Many other impurities may exist in plant waste streams besides those outlined here. Alternative treatments may be needed to remove these impurities, potentially including biological treatment methods for nutrients such as ammonia and phosphorus. Furthermore, increasingly stringent regulations are influencing wastewater treatment selection, with an increasing trend towards zero liquid discharge (ZLD). While the positive aspect of ZLD is that it eliminates all worries of impurity discharge to receiving bodies of water, the process is not to be taken lightly. ZLD can be complex, and can introduce many operating difficulties to the process. Careful planning and engineering are required ahead of any ZLD project, or any other wastewater treatment project.
1. Nowicki, H., Nowicki, G. and W. Schuliger, “The basics of activated carbon adsorption,” Water Technology, March 2016.