Using reverse osmosis and evaporation for the treatment of wastewater containing moderate levels of chlorides at an underground rock salt mining operation

Daniel J. Redetzke, Geologist                                                                March 14, 2002
Independent Salt Company
Kanopolis, Kansas

Abstract

The Independent Salt Company located near Kanopolis, Kansas, is a conventional underground rock salt mining operation.  The product, rock salt, is used for roadway de-icing, agricultural feed mixing, and hide curing processes.  Naturally occurring, saline groundwater, from various formations and rock types, seeping into two of the vertical mineshafts that access the underground workings, is the source of slightly contaminated water with moderate chloride levels.  Structures within the walls of the mineshafts, called water rings, collect this groundwater that is piped to a temporary holding tank located at the mining level.  After a brief settling period it is pumped back to the surface, where it formerly flowed into an unlined earthen lagoon for evaporation.  Through the evaporation and settling processes, both the suspended and dissolved solids in the wastewater were concentrated and precipitated onto the bottom of the lagoon as sludge.  Changes in environmental laws made the use of unlined earthen lagoons for the treatment of chloride-contaminated wastewater unacceptable. 

In conjunction with Wichita based American Industrial Water a pilot study designed to test the use of reverse osmosis for the removal of chlorides from the wastewater was conducted and yielded promising results.  The process removes chlorides and other organic salts and dissolved solids from the wastewater by allowing the passage of the water molecules but rejecting 90%-95% of the dissolved ions.  This effectively removes the chlorides from the wastewater by not allowing them to pass through the membrane.  The rejected ions become concentrated in a reject stream that is typically flushed to a drain.  In this case, the reject stream cannot be discharged as it contains high levels of chlorides and other dissolved solids.  As a result, underground evaporation experiments were conducted to determine the maximum amount of reject water, generated by the reverse osmosis process, that could be evaporated in the underground mine workings.  This information provided the design parameters for the permanent reverse osmosis system.  These experiments yielded results suggesting it is possible to effectively evaporate more than the expected amount of reject from the reverse osmosis process with little or no negative impact on the mine itself.

Introduction

The Independent Salt Company opened for business in 1913.  The original mine shaft and mill building are still in operation today.   The product, rock salt, was originally used by the meatpacking and processing industry and is now used primarily for roadway de-icing, agricultural feed mix, and hide curing processes.  During the sinking phase of the original mine shaft, attempts were made to stop the seepage of groundwater into the mineshaft as it slowly penetrated the earth punching through multiple zones of saturation.  Attempts to stop the water were unsuccessful and the only alternative was to install a collection and pumping system to continually dewater the shaft.  At some point in time, a non-overflowing storage lagoon for the wastewater was constructed on the surface.  This lagoon later became a potential source of groundwater contamination through the concentration and leaching of chlorides from the lagoon into the surrounding soils.  New environmental regulations required operators to close these earthen lined lagoons and install synthetically lined lagoons or find other alternative treatment methods.

History 

The original chloride levels in the water from the mineshafts were not recorded.  However, records from the 1970’s and 1980’s indicate chloride levels in the wastewater at that time were ranging around 5000 parts per million, nearly five times current levels.  In 1991, a letter from the Kansas Department of Health and Environment was issued to the Independent Salt Company stating the unlined earthen lagoon was no longer an acceptable method of wastewater treatment and a plan for lagoon closure be submitted for approval and alternative treatments explored.  For the next nine years, several ideas were proposed and the KDHE granted several extensions on the existing wastewater lagoon permit to facilitate the ongoing investigation of alternative treatments.  

Determination of groundwater flow rates

The first step in the serious investigation of alternative treatments was to determine the amount of water to be treated.  Multiple measurements were taken over an 18-month period to determine an accurate average water inflow rate.  These measurements were taken in the underground wastewater storage tank to determine the combined water inflow rates from both shafts as they both flow into a common tank.  The surface area of this tank was calculated and a series of depth measurements at timed intervals were taken to determine inflow in gallons per minute.  These measurements were taken from mid 2000 to present.  The total average inflow rate is 4.7 gallons per minute.

Investigation of treatment alternatives

Reverse osmosis was briefly considered in 1996 as a possible treatment alternative but, due to insufficient data on the operational and maintenance costs of such a system, was not pursued.  Consideration was given to the proposal of installing two synthetically lined lagoons on the surface for the evaporation of the wastewater at an estimated cost of $500,000.00.  After reviewing these earlier engineering reports, consideration was again given to reverse osmosis as a treatment option.  A decision was made to install and operate a reverse osmosis pilot plant to study its effectiveness and efficiency in the treatment of the wastewater.

The pilot plant was designed and built by American Industrial Water based on the specific chemical analysis of the wastewater from this operation.  It was installed, operated, and monitored over a two-month period.  The purpose of the study was to determine the effects of the high TDS water (approximately 4000 parts per million) on the reverse osmosis membranes, such as the effectiveness of an anti-scalant additive, typical ion rejection, and the maximum recovery rates possible.  The pilot plant treated the wastewater at a rate of 4.5 gallons per minute.  This system was operated for over 200 hours with promising results.  Through observation, sampling, and chemical analysis, it was concluded the reverse osmosis system could consistently remove nearly 95% of the chlorides from the water and the anti-scalant additive kept the membranes from fouling during at least 200 hours of operation. It was later found, after the installation of the permanent reverse osmosis unit, this same additive worked effectively for as long as 1200 hours of continuous operation.  Simple cleaning of the membranes with a mild acidic solution would then be required to fully restore the membranes ability to function normally.  It was also determined that nearly 70% recovery was achievable with this type of system.  The chemical analysis for the wastewater (raw water), treated water (permeate), and discharged water (blend), is shown in appendix 1.

Underground evaporation tests

In December 2000, underground evaporation of all wastewater was investigated as a possible method of treatment.  Testing was conducted in the underground mine workings to determine evaporation rates and the evaporative capacity of the mine atmosphere. 

The first experiment was designed to determine evaporation rates in gallons per square foot of surface area per day.  A plastic lined cell, approximately four feet wide by eight feet long by six inches deep was constructed of plywood, lumber, and thin plastic sheeting.  It was filled with under saturated brine water (about 85% saturated with sodium chloride) and the depth of the cell was measured periodically to determine an evaporation rate.  Table 1 shows the results from this experiment. 

Table 1

The next experiment was to determine the mine’s ambient temperature and humidity and how elevated humidity affected the mine.  Again, measurements were taken over an 18-month period using a sling psychrometer to gather data.  Average humidity was determined to be 55% and average temperature was 70.5 degrees Fahrenheit and remained very stable throughout the entire period.    Measurements and observations were also made at the intake airway, which is exposed to extreme variations in temperature and humidity throughout the year, to determine the cause and effect of condensation naturally occurring near that location.  It was discovered the extreme changes in intake temperature and humidity only affected the mine over a short distance and within that distance and beyond, humidity never exceeded 74%.  Any humidity rising above 74% immediately caused condensation of water on the surrounding surfaces, which caused the humidity to drop back to 74% quickly.  The salt seems to temper and stabilize the air after it enters the underground workings and after less than 4000 feet of linear travel the air temperature is near 70 degrees Fahrenheit and humidity is between 45% and 55% and remains there constantly throughout the year.  Condensation results in dissolution and re-deposition of salt in the same general area with no negative impact. Surface temperature and humidity does not directly affect the majority of the air in the mine.  In other words, no matter what the surface conditions are, the salt seems to keep the air within the mine stable at 70.5 degrees Fahrenheit at 55% humidity.  

During this period several evaporation experiments were conducted underground using a system of fine mist spray nozzles assembled and installed in the main exhaust airway far from the intake shaft to minimize the effects of unstable air.  This helped determine the effects of elevated humidity on the mine independent from the effects of the surface conditions.  Water was supplied to the spray system utilizing head pressure from the vertical discharge pipe in the main shaft, which contains a maximum 233 gallons of water at 365psi when full.  Water was bled back from the vertical pipe system and fed through the spray nozzle system at 100 psi using an inline pressure regulator and valve.  Humidity and airflow measurements were taken down wind from the spray area using a sling psychrometer and anemometer.  Approximate evaporation rates were determined using a psychrometric chart.  Spray nozzles with various flow rates were tested using different pressures and configurations to determine the best system to maximize evaporation efficiency. The results from one of these experiments are shown in table 2.

Table 2

Based on the data from these experiments, and considering the airflow and geothermal energy available from the surrounding rocks, it was calculated that a maximum of 1.5 gallons per minute of water could be evaporated in the underground mine workings.   Normally, the evaporation process cools air as humidity rises.  In the underground mine, geothermal energy heats humidified air as it travels past the rock effectively lowering the humidity.   This allows the same air to carry more moisture away from the evaporation area than would be the case if geothermal energy were not present.  The system is at its maximum capacity when the humidity reaches 73% at no more than 4000’ downwind from the evaporation area.  This protects the mine from condensation beyond that zone.

Also discovered during the humidity measuring process was the occurrence of natural dehumidification caused by moisture being absorbed by the surrounding salt.  Humidity measurements were taken at the base of the intake airway during cool and very humid climatic conditions on the surface.  Tests revealed that 50-degree air at 90% relative humidity quickly became 70-degree air at 45% relative humidity.  The only way this is possible is through absorption of water by the salt surfaces within the mine.  This process occurs very rapidly as the air travels a distance of approximately 4000’ in the underground workings.  Figure 1 shows one example of the effects of the mine on cool intake air.

After carefully examining all of the data, the maximum evaporative capacity of the mine atmosphere was determined to be approximately 1.5 gallons per minute.  With this evaporation rate the relative humidity in the mine would rise to nearly 73% with a dry bulb temperature of 70-71 degrees Fahrenheit. 

Results and effects of actual underground evaporation

The reverse osmosis system and the pretreatment filtration from American Industrial Water of Wichita, Ks. has been in operation since November of 2001.  A mixture of treated water and raw water has been continuously discharged on the surface since that time to simulate natural waters.  The ultra-pure permeate from the process is actually too clean to discharge according to KDHE.  Periodic sampling and testing has been conducted to ensure the discharged water meets drinking water standards.  The reject water from the process has been continuously discharged through a 10-nozzle spray system located in the underground mine workings for evaporation.  It became apparent the evaporation rate of the spray system was dependent on the amount of airflow directly through the spray mist.  The nozzles of the permanent installation are not as affective in evaporation due to localized airflow in the area of the nozzles.  The formation of a large underground pond has occurred and current humidity measurements indicate an evaporation rate of approximately 0.5 gallons per minute from the surface of the pond.  As the surface area of this pond increases, it is expected evaporation rates will also continue to increase until equilibrium with the inflow rate into the pond occurs.  Once this happens, the surface area of the pond should become static.  There have been no negative effects from the elevated humidity levels to date.

Effects of the underground ponded water

Because the ponded water is in direct contact with salt, and because the spray system is designed to spray the reject water over a pile of waste salt before it enters the pond, the water quickly becomes nearly saturated with salt.  However, once the waste salt pile has been dissolved away, the relatively fresh incoming water begins to “float” on the surface of the brine pond and slowly “cuts” into the surrounding pillars and walls of salt by the process of dissolution until it becomes saturated.  This process has been observed and is controllable in two ways.  The first way, spraying the reject water over a pile of waste salt, is the simplest, easiest to maintain, and most effective.  The second method is to install an impermeable barrier around the edge of the surface of the pond where the fresh water has a chance to come into contact with the salt.  This barrier extends several inches below the surface of the water and effectively prevents dissolution of the pillars by preventing contact with the floating fresh water.   Both methods have been tested and both seem to be effective in protecting the surrounding surfaces from damage.  There have been no other negative effects observed to date.

Appendix 1