Cost Effectively Removing Mercury in a Wastewater Streams

Authors: Bill Purves, Don Wilson, Dr. Henry Nowicki  

Introduction

A small environmental research company, Zorbtech Environmental Solutions, located in Ontario

Canada, has developed a low cost adsorbent for mercury.  This adsorbent is one of a class of

non‐activated carbons adsorbents called (THIOLS) that specifically remove Mercury and other

dissolved metals from wastewater streams.  Mercury One LTD of Twinsburg, Ohio has

independently evaluated the effectiveness of these adsorbents versus activated carbon and

sulfurized activated carbon.  The effectiveness of the THIOLS versus the carbons prompted

Mercury One to conduct one long term pilot and three bench studies for various waste streams.

The data from the studies revealed that mercury can cost effectively be removed as long as a

proper evaluation of the waste stream is conducted.  A pilot study looked at municipal waste

water treatment plant first.  Bench studies were done on a chlor‐alkali waste steam, paper mill

waste stream and scrubber waste stream from two power plants.  The data gathered revealed

that mercury in all four waste streams could effectively be removed.  The removal rate was so

highly effective that the treatment cost could be less than that of activated carbon on a cost per

gallon basis.

The Pilot Study

This pilot study was developed to demonstrate that a cost effective process to reduce mercury

effluent limits below the 1.3ng/L limit established in the Great Lakes could be done.  The facility

chosen was the Elyria wastewater treatment plant in Elyria Ohio.  The Elyria plant is an older

plant that has a secondary treatment process that discharges into the Black River which

ultimately discharges into Lake Erie.  The Elyria plant has a discharge limit of 1.3 ng/L with a

variance which allows a discharge of up to 12ng/L.  The variance was granted based upon the

previously established issue that treatment for mercury below 12 ng/L was prohibitively

expensive.  Up until the recent development of the THIOL class of adsorbent, this was true.  This

pilot study not only dispelled the cost issue but provided significant information regarding

mercury and the treatment process.

The Adsorbent

Zorbtech formulated Diosorb (a THIOL class adsorbent) specifically to remove soluble mercury

from wastewater streams. The adsorbent had been tested in the laboratory and was found

superior to activated carbon and sulfurized carbon which has been the adsorbent of choice.  In
bench tests, the Diosorb was more than ten times more effective than sulfurized carbon and

more than 100 times more effective than activated carbon.  Bench tests are useful but real

application is critical.

Effluent Streams (Some History)

All adsorbents treat dissolved mercury.  Mercury that is trapped on to solids and suspended

solids in the water through the natural plant process are not removed by any adsorbent except

by physical separation.  Mercury One tests the discharges from hundreds of wastewater

treatment plants.  The key issue has been is the level of suspended or total solids found in

effluent samples.  Though this study did not quantitate the levels of total solids or suspended

solids in various discharges, the study did account for that problem.   

The level of dissolved mercury also varied based upon the condition of the effluent in terms of

pH and influent conditions.  Some facilities have a relatively low influent mercury concentration

and a resulting low discharge level of mercury.  Some facilities have a higher influent total

mercury concentration and a resulting higher level of effluent mercury discharge.  The

traditional process for lowering mercury effluent limits has been to lower influent levels.  This is

usually done by an aggressive pretreatment program but works to a limited extent.  

Some of Mercury One’s clients have reduced their effluent mercury discharge limits by reducing

the influent level but have not achieved the 1.3 ng/L limit in the Great Lakes.  The number of

wastewater treatment plant clients that achieve the 1.3ng/L limit on a routine basis is less than

10% of the over 200 clients that Mercury One serves in the Great Lakes region.  Though lower

influent mercury levels results in lower effluent mercury levels may be common, there are

some plants that have significantly lowered their influent mercury levels and still have not

achieved the 1.3ng/L Great Lakes limit.

Secondary versus Tertiary Treatment

The Elyria plant is a secondary treatment facility and was chosen based upon the nature of its

discharge and the historical data accumulated by Mercury One.  Based upon Mercury One data

and Elyria’s willingness to allow the pilot study this became the facility of choice.  The average

suspended solids content leaving the facility was 10‐8 mg/L (based upon plant knowledge).  

Another local treatment plant that has tertiary treatment and has a lower suspended solids

content (8‐5mg/L) based upon plant knowledge.  Both plants have the same average mercury

discharge levels of 3 ng/L.  Some other treatment facilities amongst Mercury One clients have

secondary or tertiary treatment and meet the 1.3ng/L limit a majority of the time but have

lower influent mercury.  There are other factors that contribute to these variations but a

majority of the time higher suspended solids means higher effluent total mercury.
The Pilot Plant Concept

The initial concept of the study was to determine the capacity of Diozorb in a real working

environment.  Mercury One determined that at the Elyria plant a majority of the mercury in the

effluent was in the solids.  Based upon their own data of comparing total and dissolved solids

for other clients this would normally be true.  However the mercury content dissolved in the

Elyria effluent was always above the 1.3ng/L limit regardless of suspended solids content.  In

cases when the historical samples looked nearly clear, (almost no visible solids) the total

mercury level was not necessarily lower than those with higher suspended or visible solids.  The

pilot plant was purposely designed to provide tertiary filtration as well as mercury adsorption.

The plant consisted of two treatment tanks.  The first tank removed solids down to <5 microns.  

The second tank contained the adsorbent.  Three different trials were started and data

collected, however the first two trials ended in equipment failure even though all final

discharges were less than 1ng/L.  During these first two trials more than 19,000 liters of water

passed through the two tanks.  After a reconfiguration and design improvements, the third trial

was started. The trail lasted 11 days.  All Samples were collected by Elyria personnel.  The Elyria

chemist analyzed for other metals while Mercury One analyzed only mercury.  The mercury

levels were consistently below 1.3ng/L during trials 1 and 2.  The third trial started out in the

same manner.  For the sake of brevity, only some of the data from trial three is shown below.

The system was set up as follows:

The effluent from the facility’s final outfall was pumped by the facility’s pump to the first sand

filter tank.  The discharge from the tank (<5 micron solids and dissolved Mercury) was pumped

to the adsorbent tank.  The discharge from adsorbent tank flowed by gravity to the drain.  

Samples designated pre‐filter were from the final outfall before entering the first tank.  Samples

designated as post filter were samples discharged from the adsorbent tank.  The data is as

follows:

Mercury Concentrations

Date    Pre Filter    Post Filter   

6/29/12 2.25ng/L    <0.5ng/L

7/6/12    8.85ng/L    0.92ng/L

7/9/12    32.9ng/L    1.04ng/L

7/11/12 54.3ng/L    0.94ng/L

The test was terminated no 7/12/12 due to equipment failure on the primary tank.  No further

samples were taken. Elyria did not provide suspended solids levels with this data.  The plant

chemist did confirm that the suspended solids content on the final day was higher than normal.  

The capacity of the adsorbent was never reached (no breakthrough) because of the equipment
failure.  The volume of water pumped through the system was 267000 liters.  The Mercury level

never exceeded 1.3 ng/L.  The quantity of Diosorb used in the study was 400 grams.  Based

upon the cost of the adsorbent, the cost to treat was $0.000012 per liter.  The adsorbent cost

for this 6,000,000gallon per day plant would be approximately $300.  This is a manageable

material cost for most treatment plants.

Redesign and Flexibility

The two tank system provided not only some significant answers to the treatment of solids and

mercury levels.  The system provides flexibility.  Facilities that have only secondary treatment

may need to add tertiary treatment to remove as much solids as reasonably possible.  The

redesign uses only one pump and gravity treatment after that.  The adsorbent tank only

requires a stirring process and then gravity feed to the discharge point.  The new system has

sampling port is added between the pre‐treatment tank and the adsorbent tank to test the

level of mercury before adsorbent treatment.  Thus the level of mercury removed by simple

filtration can be studied.  The adsorbent tank does a final “polish” and the mercury meets the

Great Lakes 1.3ng/L limit as in the initial pilot test.   

If a plant already has tertiary treatment, the adsorbent tank is the only system needed.  This

“polishing unit” becomes the only equipment needed for testing.  The system is designed to

treat up to 1 million gallons using 2‐4 kilograms of resin based upon flow requirements.  Thus it

becomes a very cost effective pilot test with results that can be easily engineered for flows over

12 million gallons per day.

Bench Studies

Three other bench studies were conducted on volumes of 40 to 200 liters of water.  In each

study a two or three step system was employed.  Prior to each setup the waste water was

evaluated for solids content and type and dissolved versus total mercury content.  In each study

1 to 10 grams of adsorbent was used.  The quantity used was based upon the treatment tank

volume not adsorbent capacity.  The ultimate goal was to develop cost effective removal not

just removal.

Bench Study # 1

Chlor‐Alkali Plant

Mercury Levels entering the waste water treatment system at the plant average 1.6 mg/L.  The

system treats the mercury by precipitation with sulfide.  The resultant clear liquid layer is then

passed through activated carbon and sulfurized activated carbon.  The final outfall contains

between 500 and 5000ng/L Mercury.  This outfall is then mixed with other water to achieve an
average of 25ng/L.  Zorbtech was requested to attempt to reduce the mercury concentration to

under 100ng/L.   The original column pilot study did not consistently meet the requirement.  

Mercury One evaluated the situation and determined that the excess sulfides in the water were

preventing the adsorbent from removing the mercury effectively.  After some study, Mercury

One and Zorbtech developed a bench scale system to remove the mercury without the use of

sulfides.   

Design 1

Using only adsorbent, the mercury level was reduced to 7.8ng/L using a four tank system.  The

system required settling the solids out using a standard settling tank first.  The second tank

contained adsorbent at a 1% ratio by weight of adsorbent to water.  The third tank contained

the same ration and the final tank contained a 0.1% ratio.  This setup was effective but very

short lived.  The conclusion was that this system would cost over $20MM (USD) annually to

operate for a 300 liter/ minute outfall.  Thus a second design was created.

Design 2

The wastewater with the 1.6mg/L concentration was stoichometrically mixed with stannous

chloride and purge via air of the mercury.  The mercury vapor was transferred to a Iodated

carbon bed where it was captured.  The remaining liquid was neutralized to a pH of 7 where the

stannous chloride precipitated out.  The neutralized water was then passed onto the adsorbent

bed where the remaining Mercury was removed by the adsorbent.  The final outfall was 4.7ng/L

of Mercury.  This setup had more promise as it was estimated that the system would cost

$3MM annually in chemical and adsorbent costs.  A pilot study for this plant was never setup

because the facility was granted a variance by the regional authority to continue polluting the

river with an estimate of over 2.4 kg of mercury per day.

Bench Study #2

A paper producer requested a proposal from Zorbtech to reduce their Mercury concentrations

to under 12ng/L.  The facility has an extensive wastewater treatment system that can remove

the solids but not the organic matter and dissolved mercury.  When using the adsorbent to

remove the mercury, the capacity was very poor.  To improve the removal the organic matrix

needed to be reduced.   

Mercury One developed a process to remove the organic matrix to a level that allows the

adsorbent to cost effectively remove the mercury.  The final discharge level remained a

consistent 9.2ng/l or less.  An engineering scale up of the bench study demonstrated that the

treatment system to treat an outfall at 7000L/minute would cost $4MM annually in adsorbent
and chemical costs.  This producer is looking at a pilot plant study to verify the technology and

develop a full scale plant.

Bench Study #3

This client is a power producer.  The issue is the high levels of Mercury being discharged from

the scrubber waters.  Zorbtech and Mercury One received 100 liters of the water to study at a

bench scale that would eventually evolve into a pilot plant and then scaled to a full plant.  

Originally the focus was on the final outfall which ended in a treatment system that is similar to

the wastewater treatment plant presented in the first pilot study.  This study and achieving

discharge levels of less than 9ng/L was not a problem even at a 45MMliter per day discharge.  

The real challenge was treating the scrubber water directly.

If State and Federal authorities want the reduction of mercury to occur at the scrubber

discharge before entering the ponds for mixing and reduction, the approach has the benefit of

consistency.  As part of this study, a process was developed that effectively removed most of

the mercury in the scrubber water prior to discharge into the settling ponds.  This approach will

need to be studied further to evaluate the cost effectiveness of the process.  The process did

generate levels of less than 8ng/L and could reduce the levels even lower consistently.   

Some coal fired power plants have treatment systems that effectively remove mercury to

discharge levels of less than 500ng/L.  The use of sulfides in these processes limits the reduction

level.  The scrubber water in this commonly used setup is reduced to 200‐700ng/L before

mixing with the cooling tower water and discharged into the river.  The issue is that this

discharge is rarely under 12 ng/L of Mercury and is very inconsistent.  By utilizing a three tank

system, Zorbtech and Mercury One reduced the mercury to less than 24ng/L before discharge

into the ponds. This required removing sulfides and ferric chloride in the first tank and then

using the second two tanks with adsorbent to remove the dissolved mercury.   

As in all of the other studies, the highly efficient adsorbent removed mercury cost effectively.

The water must go through pretreatment to achieve the ultimate result.  The cost of chemicals

and adsorbent in this study based upon a rough engineering scale up was between $3MM and

3.5MM annually.  This system would remove 0.5 Kg of mercury daily from the environment

based upon the original data provided by the client.

Conclusion

When evaluating a municipal wastewater treatment plant based upon adsorbent cost alone and

the simplicity of the system, it is estimated that the treatment cost with the Diosorb is

approximately 0.00005$ per gallon of water.  The new family of adsorbents, (THIOLS) in bench

scale studies demonstrates a real potential for cost effective removal of mercury from
wastewater.  The implementation of such adsorbents requires a different approach that may

require limiting sulfides as a primary pretreatment and using more sophisticated physical

removal processes.  The adsorbents are very effective in isotherm tests but have yet to be

tested in enough real world pilot studies to have accurate cost benefit analysis data that is

completely reliable.  The Elyria plant experience was the first true pilot study in a real working

environment that provided insight as to the cost effectiveness of the new family of adsorbents.

The bench scale and pilot plant results strongly indicate that mercury can be removed cost

effectively.  What are missing are enough studies on a pilot level to verify the bench data and

generate reliable treatment costs using the Diosorb and the related family of adsorbents.   The

Elyria pilot study demonstrated that mercury could be removed cost effectively based upon the

adsorbent capacity.  The system used was basic off the shelf equipment that would be used by

any treatment plant.  The difference is that the approach was focused on the adsorbent and

pretreatment to effectively use the adsorbent.   

All of the cost estimates were exclusive of capital and operating costs, but the Diosorb types of

adsorbents were the focus of this study.  The first full pilot study provided enough data that

mercury can be cost effectively removed from wastewater as long as the focus is on maximizing

the effectiveness of the adsorbent.  More large scale pilot studies are crucially needed to prove

that Mercury can be cost effectively removed using the THIOL family of adsorbents.