National effluent treatment standards are set by the Environmental Protection Agency (EPA). These limits form, in most cases, the baseline treatment technology required for compliance. However, state and local regulators are empowered to enforce equal or stricter standards depending on the local conditions.
What most perplexes wastewater operations is the ever-present probability of regulatory change. Regulatory change is inevitable, and smart managers will continually monitor advancing technologies and plan for the day of their ultimate implementation.
However, because most national standards are technology-based, the standards necessarily tighten as technologies improve. For example, EPA recently announced its intent to widen the rules for toxic metal discharges. The move would regulate for the first time the millions of pounds of arsenic, mercury, selenium, lead and similar pollutants released annually in wastewater streams. The reason: Current regulations, which have been in place since 1982, have not kept pace with technological improvements.
The Effects of Regulatory Change
Regulatory change closely mirrors advancements in technology as well as the ability to monitor such advancements. Going back 40 years, clarification using lagoon-type processes were the common practice. These manmade lagoons were excavated in low-permeability strata with or without liners, and with or without aeration. Lagoons remain a liability because there is always a potential for overflow, groundwater contamination and structural failure.
By the mid-1980s, measurements for critical contaminants began approaching parts-per-billion. Lagoons were gradually replaced with more sophisticated biological and physical-chemical treatment processes that dramatically improve removal efficiency.
Today we have the technology to achieve parts-per-trillion (ppt) effluent capabilities. To attain these ultra-low limits, both the insoluble and soluble portions of contaminants now have to be reduced. New equipment designs such as specialty resins for advanced ion exchange have been developed to target single contaminant offenders such as mercury, chromium, selenium, arsenic and boron.
Complicating matters is that concerns for heavy metals and contaminants vary by location. Certain portions of the country are concerned about the high total dissolved solids discharges associated with extensive drilling for natural gas embedded in shale formations, while in other parts of the country mercury is of particular interest because of its high toxicity and the fact that it bio-accumulates. As a result of these concerns, water-quality based standards such as initiatives in the Great Lakes and Chesapeake Bay are seeking effluent limits as low as 1-ppt to 10-ppt.
However, notwithstanding promising technologies, there remains extreme difficulty in achieving single-digit parts-per-trillion levels. Furthermore, from a health risk perspective, heavy metals such as mercury have no minimally-defined non-cancerous threshold. Hence, even being exposed to a wastewater stream reduced to parts-per-trillion of mercury may still be harmful to the individual and the environment.
Increasing Complexity of Treatment Options
The complexity and cost of compliance has reached a point where facility operators simply want to exit the wastewater treatment-monitoring business and implement one treatment system that can do it all. Zero liquid discharge (ZLD) is a concept is that could be particularly beneficial in areas of the country where water is scarce and water reuse is increasingly necessary.
ZLD is a process that removes dissolved solids from wastewater and returns distilled water to the process (source). Falling film evaporation is an energy-efficient method of evaporation used to concentrate the water up to the initial crystallization point. The resultant brine then enters a forced-circulation crystallizer, where the water concentrates beyond the solubility of the contaminants and crystals are formed. The crystal-laden brine is dewatered in a filter press or centrifuge and the filtrate is returned to the crystallizer. The collected permeate from the membranes, the falling film evaporator and forced-circulation crystallizer distillates are returned to the process for reuse, thus saving on water resources while eliminating liquid discharge.
This process may utilize all or part of multiple treatment zones.
Zone 1: Pretreatment. In many cases, it is more cost-effective to remove contaminants such as total suspended solids, hardness and silica prior to evaporation and crystallization.
Zone 2: Membrane Filtration. Where possible, membrane filtration such as reverse osmosis can be used to treat the wastewater. The permeate (clean water) is reused in the process and the reject/concentrate is sent on to evaporation.
Zone 3: Preconcentration/Evaporation. When a significant amount of water needs to be evaporated prior to the crystallization step, pre-concentration in a falling film evaporator is the most efficient solution. These evaporators require less heat/power per unit of water evaporated.
Zone 4: Crystallization. The crystallizer is the heart of the ZLD process. Typically, forced-circulation crystallizers are used to evaporate the water past the crystallization point. Crystals are mechanically dewatered, and the resulting filtrate is returned to the crystallizer. The crystallizer usually requires corrosion-resistant materials due to the higher salt concentrations present. In some cases, part of the crystallization can be achieved by spray driers to overcome high solubility of certain salts. The condensate is returned to the process for reuse. This crystallization process is extremely sensitive to the wastewater chemistry, as the ions present will determine the boiling point elevation, which impacts the power consumption.
Zone 5: Dewatering. Sludge generated by Zone 1 is generally dewatered in a filter press. A dry solids concentration of 20 percent to 50 percent can usually be achieved, and the filtrate is simply recycled back to the beginning of the pretreatment system. The crystals from Zone 4 also can be dewatered, but corrosion-resistant materials are usually necessary due to the high salt concentrations present.
Multiple Effects. Evaporation processes can be installed in series such that the vapor from one is reused in the next. In this way, the efficiency of the evaporation process can be increased by 100 percent to beyond 300 percent based on the number of evaporator effects installed. This increases the capital cost of the system, but it is more economical for larger-flow operations considering the energy saved.
Waste Heat Usage.Economics are enhanced when waste heat found in many applications can be productively reused in the ZLD design. This can take the form of dryer exhaust gas or low-pressure return steam.
Notwithstanding regulatory pressure or the need for water reclamation and reuse, the case for ZLD also is made by economic analysis. Compliance with discharge limits comes with a quantifiable cost in both capital and operating expense. This cost of compliance is continuously increasing as limits grow more stringent. Installation of ZLD technology also incurs measurable costs. However, this cost is unaffected by effluent limits. At some point, compliance will necessarily become cost prohibitive when compared with eliminating the discharge of wastewater altogether.
ZLD will become the next wastewater treatment advancement as water scarcity and reuse grow and regulatory pressures will inevitably lead to ever lower effluent limits and more complicated treatment technologies. Eventually, this trend will help transform wastewater treatment and discharge into complete water reuse management without wastage or concerns for the environment.