Implementing Integrated Zero Liquid Discharge

Reviewing results from 11 installations shows what can be achieved
By Todd Potas | May 13, 2011

There are many technological solutions available to help ethanol plants achieve zero liquid discharge (ZLD). Traditional ZLD systems use brine concentrators and evaporators for the undesired, salt-laden cooling tower (CT) blowdown and reverse osmosis (RO) concentrate, which is evaporated and the solids hauled off-site to solid waste handling facilities. Some permits allow discharging of CT blowdown and RO concentrate in the nearest flowing river or stream.

U.S. Water Services has designed ZLD systems at more than a dozen ethanol plants in the U.S., trademarking its integrated system iZLD. Some implemented ZLD during initial construction; others transitioned an existing facility to ZLD or sought to optimize an existing ZLD. The facilities analyzed in this article demonstrate CT blowdown and RO concentrate can be successfully integrated into the ethanol facility process while maintaining process water quality compatible with biological processes, cooling tower assurance and coproduct quality. Ongoing monitoring, however, becomes extremely important because a poor trend, such as high chlorides or biological fouling, can take months to correct, even leading to plant shutdown.

Many ethanol facilities are required to have a National Pollutant Discharge Elimination System permit for industrial non-contract water discharge. The renewal process typically occurs every five years, which allows the regulatory agencies to review and update permit limits and/or provide additional protection for degraded or sensitive surface water resources. In many instances, these renewals result in new discharge limits that facilities cannot achieve without retrofits. As soon as a facility has a discharge limit that is more stringent than a contaminant parameter level in the incoming water, ZLD becomes a very viable alternative. For example, if a facility receives a conductivity limit of 1500 µmhos/cm (micromhos per centimeter) for their water discharge permit, and the conductivity of the incoming water is the same value, discharge no longer benefits the overall water balance for a facility.

Typically, water is obtained from on-site wells, surface waters or municipalities. Water leaves the facility through evaporation, in the distillers grain, discharged wastewater streams or liquid, or solid waste hauled off-site or pumped to evaporation ponds. If regulations require the discharge to be as good as, or better than, incoming water, re-use of the water simply makes more sense than using more raw water and continuing to discharge under a restrictive and burdensome discharge permit.

Quality Parameters
Looking at a few important water quality parameters, such as conductivity, sulfate and chloride, reveals general trends and suggests what process changes may be necessary to implement an iZLD scenario at a new or existing facility. Table 1 shows the average across 10 ethanol plants for water quality and performance parameters for discharge water, as well as projected iZLD performance and actual results. The 10 dry mill corn-ethanol plants analyzed produce between 50 and 120 MMgy. Table 2 shows the range in raw water quality at the 10 plants.

The iZLD system utilized by these 10 plants improved chloride levels in the process water as shown in Table 1. The summary data shows that chloride-contributing chemicals, such as hydrochloric acid or chloride-containing biocides like bleach, were replaced or eliminated. If chloride is allowed to cycle up, corrosion in the plant process equipment and piping will likely result.

Table 1 also shows the iZLD operations tended to concentrate sulfate levels in process water when compared to operating with discharge. Unlike chloride, sulfur cannot be readily reduced or eliminated. Therefore, control and management of sulfate levels becomes more significant. Encouragingly, increased sulfate levels in water do not result in a significant increase in the sulfur content of DDGS, indicated by the sulfur levels for DDGS in Table 1 that averaged a projected value of 0.06 percent.

The average conductivity levels in the 10 plants also show a concentrating trend once iZLD is applied, requiring ion-specific, plant-by-plant review to determine where and how much the concentration of conductivity-contributing ion levels can be tolerated, as well as the chemistry controls required.  The problematic ions can include sodium, silica and hardness, which can increase fouling or corrosion. 

In addition to water quality issues, changes to air quality must be considered when planning iZLD systems. The levels of collective total dissolved solids (TDS) are often limited for the cooling tower exhaust, commonly stated as drift loss. Evaporated solids from the cooling tower are particulate emissions generally subject to regulation, and any increase due to ZLD may require modification to a facility air permit.

Plant Conversion
Once regulatory requirements are completed, including any needed permits, equipment installation or modifications can begin. In virtually every instance of iZLD installation reviewed here, it makes technical and economic sense to replace sulfuric and hydrochloric acid with plant-generated CO2 for recarbonation (pH adjustment) of the treated plant water. This replacement is critical for managing process water ion balance, which can include sulfate loading in the DDGS. The recovered CO2 can also be used for acidification in other areas of the facility.

The conversion to iZLD begins with integration of the two primary wastewater streams, beginning with cooling tower water. Once the CT blowdown is integrated and observations of ion cycling in the water system at all stages stabilize, RO concentrate is introduced. Integration is typically complete in one or two weeks, depending upon site-specific issues. Subsequent water testing, both on-site and with samples sent off-site for more detailed lab analysis, monitors plant operation. Water specifications monitored include cooling tower makeup, boiler water makeup, and most importantly, process water used in fermentation.

While the 10 facilities reviewed using iZLD have achieved satisfactory performance, there are examples where other iZLD systems are needed. One iZLD facility with an evaporator/crystallizer and storage pond was reviewed, and its water quality parameters given in Table 3. The raw water quality is very challenging in this example, showing a clear advantage to using an evaporator/crystallizer and brine storage pond to handle the high salts. While there was some cycling up of the key water quality parameters, concentrations of conductivity, sulfate and chloride did not reach the levels demonstrated for the averaged iZLD facilities. In addition, relatively good water usage performance was achieved at 2.81 gallons of water/gallon of ethanol produced and the projected versus actual results were fairly consistent.

Total Water Savings
Facility water use efficiency performance was an average of 4.3 gallons of water/gallon of ethanol produced for all the facilities reviewed prior to ZLD implementation. The projected, worst-case water use after going to ZLD was an average of 3.1 gallons of water per gallon of ethanol produced, or close to 1 billion gallons of water per year on the combined 800 MMgy ethanol production of all plants reviewed here. The actual performance was closer to 2 billion gallons of water saved per year. The large difference between projected and actual performance for water use per gallon of ethanol occurs often, primarily because the projections are done on a worst-case water balance, raw water quality and supply information. Many facilities were able to optimize in-coming water among multiple wells, municipal supply, and/or graywater and stormwater, both on-site and off-site. The relatively soft water properties of storm water can be very favorable to any ZLD operating scenario, especially if a facility already has a pond for containment and control of stormwater.

While projections for water use per gallon of ethanol differ greatly from actual performance, this review of the conductivity, sulfate and chloride levels for the plants analyzed demonstrates good agreement between projected and actual results. Based on these results, facilities contemplating an iZLD system can predict the likely operating differences with some level of confidence. 

The financial and environmental liability benefits of not discharging can be significant. Monitoring, testing and reporting of discharges for permit compliance have become much more costly in recent years. Requirements for whole effluent toxicity (WET) testing, for example, can greatly increase costs. Many facilities have, or are considering, voluntary ZLD conversion, which can permit optional discharge of nonprocess water and stormwater. If no discharge occurs, reporting can be as simple as stating “no discharge” in monthly NPDES discharge monitoring reports. No sampling and analysis costs would be required, and no potential discharge permit violations would occur.

Each facility location will have unique advantages and disadvantages that require a customized engineering approach to optimally design a ZLD system that accounts for local water quality and quantity, as well as process and operational variations in the ongoing effort to conserve water usage.

Author: Todd Potas
Biofuels Business Leader, U.S. Water Services