Review of URS document
Passive And Semi-Active Treatment Of Acid Rock Drainage; State Of The Practice, April 2003

By Step by Step, June 1, 2003


SUMMARY OF DOCUMENT:

This document presents an “overview of current state of the practice applications for passive and semi-active treatment systems to treat acid rock drainage (ARD) associated with metal mines.” A stated limitation of the study is that although they spoke with system designers and operators of passive treatment systems, usually URS was unable to obtain information on why the systems failed, either because an evaluation had not been done, or the information not available.

The study reviewed 7 passive treatment systems: aerobic wetlands, anoxic limestone drains (ALD), open limestone channels (OLC), settling ponds, successive alkalinity producing systems (SAPS), solid-reactant anaerobic or sulfate reducing bacteria (SRB) bioreactors and liquid-Reactant SRB Bio-reactors. The liquid SRB is a semi-active system that requires inputs of two liquids regularly. The study involved a literature review that included other more comprehensive literature reviews including a 1996 paper from Canada that reviewed over 100 papers.

URS reviewed 5 case studies using SRB’s. There were no case studies for treatment methods other than SRB’s. The Elizabeth Mine has flows of up to 2000 gpm (SLERA). The majority of the load to the WBOR is iron and aluminum, and the water is acidic. Previous studies have designed for flows of up to 135 gpm from TP1. The 5 case studies in general did not address sites with similar chemistry and flows. The 5 case studies reviewed were:

1) The Burleigh tunnel treated 7 gallons per minute (gpm) of neutral pH water. These SRB’s failed due to spring runoff which ‘appeared to have changed the microbial ecology of the reactor”.

2) The Brewer Pad 5 operated for 18 months, design flow dropped from 5 gpm to 0.75 gpm (due to changes in water chemistry), and after maintenance and re-building due to invasive plants, it operated effectively for another 3 months until it was closed.

3) The Ferris-Haggerty mine has neutral pH and high copper concentrations. A pilot cell treated 3 – 5 gpm for 2 years, and it was de-commissioned.

4) Doe Run mine treated 1200 gpm of high pH (8) water with high lead concentrations since 1996. The substrate in the SRB’s has been completely removed and replaced twice.

5) A liquid SRB at Leviathan Mine treats up to 16 gpm of pH 4 water with relatively low concentrations of copper, nickel and zinc (1 mg/l each), 100 mg/l of iron and 25 mg/l of aluminum. The water from TP3 has a similar pH, but has higher average concentrations of copper (21 mg/l), zinc (5 mg/l) and aluminum (30 mg/l) and higher flows.

The study summarizes the geology and geochemistry of the Elizabeth Mine and presented both a conceptual design for treatment on TP1 and TP3.

The conceptual plan for TP3 recommends an open limestone channel (OLC) that flows to oxidation and settling ponds to remove iron and aluminum, which lead to a liquid sulfate reducing bacteria (SRB) system to remove heavy metals and a final step of aerobic wetlands.

The conceptual plan for TP1 recommends an anoxic limestone drain (ALD) leading to oxidation and settling ponds to remove iron and aluminum and aerobic wetlands with an algal mat.

The study included a literature review of costs and an initial cost estimates for both conceptual plans. TP1 has capital costs of $268,500 and TP-3 has costs ranging from $1,400,000 to $1,700,000 for solid and liquid SRB’s respectively. Operation and maintenance costs are estimated at $48,500 annually for TP1, and $120,000 - $132,000 for TP3.

General comments:

1. Most case studies did not address flows or chemistry that are similar to E-Mine. Particularly waters with high iron and aluminum concentrations and variable flows.

2. Many case studies reviewed were for pilot projects and/or short periods of time and only reviewed systems using sulfate reducing bacteria systems (either liquid or solid).

3. There were no reviews of case studies of other passive treatment systems (ALD’s, SAP’s, OLC’s, wetlands etc.)

4. Conclusion on a strong recommendation of liquid sulfate reducing bacteria systems (SRB) seems premature. The data is insufficient to strongly recommend the technology. The literature referenced note that SRB’s are not tested for large sudden changes in load, flow, concentration and temperature (similar to E-Mine). The liquid SRB is a semi-active system that requires alcohol and sodium hydroxide at a combined cost of $1 for every thousand gallons treated.

5. State of the practice review neglected to look at ‘aluminator’ flushing system, or constructed wetlands (combined aerobic and anaerobic wetlands). There are gaps in literature review.

6. Oldest SAPS installed prior to 1991, have 12 years of data or more, no reviews of long-term systems.

7. The “treatment train” approach taken in both conceptual plans is a good one. Aluminum is easily precipitated by raising the pH over 4.5, iron can be precipitated out by exposure to oxygen if there is enough alkalinity to maintain a pH higher than 6. These two metals are the major part of the load to the West Branch of the Ompompanoosuc. The metals (copper, zinc….etc) can be removed in wetlands, SAPS, ALD’s, SRB’s or by other treatment methods.

8. Treatment recommendations for TP1 and TP3 do not agree with the Figure 1 flow chart, based on flows and chemistry of E-Mine (see below).

9. The chemistry and flows of the E-mine do not appear to support the use of ALD’s. OLC’s may work for conveying water, but will probably not do much for treatment due to the low slopes and inability to flush. SRB’s, are best used after iron and aluminum are removed from the flows. Initial treatment would be with SAP’s and aluminators.

CONCEPTUAL APPROACHES FOR TP1 AND TP3.

The conceptual approaches to treatment do not agree with the flow chart on Figure 1.

TP3

TP3 has water with lots of oxygen (DO >5 mg/l), flows of <200 liters per minute, pH of <4.5. Figure 1 would recommend an anaerobic wetland or SAP’s for treatment based on this chemistry and flow. Anaerobic wetlands require a larger land area for treatment than SAPS.

There are problems with both the OLC’s and liquid SRB’s recommended for this site. As URS states, OLC’s are best on slopes of 45 – 60%, or according to the literature, slopes that exceed 20% where suspended sediment can be kept in suspension and precipitates flushed from the system to keep the limestone from being armored. They do not provide treatment on slopes that are less than 8%. Although some treatment occurs even when the limestone is armored. Channel slopes and flows in the upper watershed are not sufficient to allow the exposed limestone to self-scour.

The liquid SRB’s are microbial systems that have not been tested for and have been found to fail with changes in flow, chemistry, temperature and load as are found at the Elizabeth Mine. SRB’s are a better technology for dealing with metal acidity (like from copper and zinc), although they can treat iron and aluminum, there are better technologies for removal of iron and aluminum.The changeable flows and chemistry of TP3 and the flow chart do not seem to support the conceptual plan for remediation.

TP1:

The conceptual plan for TP1 seems to address seeps only. Seeps are a relatively small and steady flow, but the streams below TP1 are a bigger problem, and during most months are the primary source of loading to the WBOR. There is much more water and load below TP1 than just the seeps. (If seeps are defined as small seeps at base of TP1 with high concentrations of reduced iron, low dissolved oxygen and relatively steady low flow).

WON'T DIVERSION GREATLY AFFECT THIS, REDUCING THE FLOW AND METAL LOADING IN THE STREAM?

The plan for TP1 needs to address the major source of load. High iron and aluminum are the primary contributors to acidity d/s of TP1. Treatment downstream of TP1 will probably need to treat relatively large volumes of water with high concentrations of dissolved oxygen, iron and aluminum. It is critical that the design address variable flow, as well as varying load, chemistry and temperature. The dissolved oxygen concentrations in streams below TP1 were quite high (over 8 mg/l). The installation of channel diversions, the horizontal drains at the base of TP1 and the removal of the pond on TP1 will effect both the chemistry and flow of the surface water downstream of TP1. Surface water flows will be reduced, and the seeps could change chemistry due to reduced residence time within the pile and reduced recharge because the pond will be drained and a cap of some sort put on the pile. Both flows and metal loading will be reduced by the three actions mentioned (diversion, draining of pond, installation of horizontal drains.)

ALD’s function with anoxic water with dissolved oxygen concentrations of <2 mg/l. The many small seeps at the base of TP1 are anoxic and need to be collected without exposing to oxygen. ALD’s are prone to clogging if aluminum concentrations exceed 25 mg/l and can clog with even very low aluminum concentrations (1 mg/l). Skousen states: “Longevity of treatment is a concern for ALD’s especially in terms of water flow through the limestone. If appreciable dissolved iron and aluminum are present, clogging of limestone pores with precipitated Al and Fe hydroxides has been observed. The maximum alkalinity that ALD’s can generate is about 300 mg/l” The groundwater seeps at the base of TP1 had a measured acidity of 1900 mg/l in calcium carbonate equivalents. ALD’s are likely to clog with this chemistry, and not function well because of the high acid neutralizing capacity required. At the Elizabeth Mine, ALD’s do not have the capacity to generate enough alkalinity to treat the flows. Acid mine drainage (AMD) that does not contain high concentrations of iron and aluminum can be effectively treated with ALD’s.

Figure 1 recommends open limestone channels for this location, but this is not appropriate technology due to usual lack of high enough flows and the low slopes.. Figure 1 recommends anaerobic wetland or SAPS as treatment alternative for chemistry of streams downstream of base of TP1.

CHANGES IN CHEMISTRY/FLOW

It is important to remember that the chemistry and flow will change with the removal of the pond, the installation of the horizontal drains in TP1 and the construction of diversion ditches. Upon removal of pond, more iron will stay in suspension in the stream, as it will not be able to precipitate out into the pond. The proposed systems should be designed for the flows and chemistry specific to the area. Diversion is the first step that will change flows and chemistry.

ECONOMICS

Economics should be re-worked with appropriate designs for each area. The selection of treatment systems is not consistent with Figure 1 on the basis of the chemistry and flows of the E-mine. Capital costs are low for the seeps at TP1 (unless it is only to treat seeps immediately at base of TP1 and not surface water). The semi-active treatment system proposed for TP3 is very expensive.

Figure 1 recommends SAP’s or anaerobic wetlands. Capital costs using SAPs as part of treatment trains for TP1 and TP3 were calculated in 1999 and found to be $750,000 $950,000 with estimated annual operation and maintenance costs of $11,000 $17,000 for TP1 with a design flow of 125 gpm. TP3 had capital costs of $245,000 $299,000, with estimated annual operation and maintenance costs of $4,000 $8,000 For tp3 with a design flow of 35 gpm and design lives of 20 years. These costs are in 1999 dollars and would need to be updated with changes in both design, and capital and O and M costs.

The cost estimates in the report are for systems that are inappropriate according to Figure 1 of URS’s report. They should be refined as design is refined.

MISSING INFORMATION

Two passive treatment systems were not reviewed:

1) Constructed wetlands that combine both aerobic and anaerobic treatment
The Canadian literature review that was referenced examined constructed wetlands. Constructed wetlands combine both aerobic and anaerobic processes. In the aerobic portion, they remove metals either by precipitation, attachment to another compound or exchange with another compound (one ion replaces another on a molecule). In the anaerobic portion, sulfate reducing bacteria and chemical and micro-biological activity in the underground limestone beds are used to reduce acidity.

2) A design that combines the use of successive alkalinity producing systems (SAPs) with a flushing system to remove precipitates from the system called the “aluminator” by the designer of the SAP’s (Kepler and McLeary).

SAPs are intended to generate alkalinity and increase pH. SAP’s treatment is in an anaerobic environment. Iron is removed in a downstream oxidation and settling pond. Aluminators© are similar to saps, but intended to precipitate the aluminum within the limestone of the (modified SAPs). The periodic flushing of the aluminum precipitates from the system will keep the SAP from being plugged. Experience in Oven Run, Pennsylvania shows that “Long duration, high volume flushing performed on a regular schedule has maintained hydraulic conductivity through the Oven Run Site F system. The system has continued to produce high quality water since construction” (Milavec and Seibert, 2002). The use of SAP’s combined with an aluminator, or similar system to remove precipitated metals should be reviewed. The literature review mentions the need for quarterly flushing, but says initial monitoring should be done during the first year to determine the operation and maintenance schedule.

CASE STUDIES OF SAPS/ALUMINATOR

Although the literature reviewed contained case studies of SAP’s from the references (Skousen), this was not mentioned in the report. A 2003 paper by Damariscotta, the inventor of SAP’s was not referenced in URS’s review. It is available on-line and was presented at the 2003 West Virginia Surface Mine Drainage Task Force Symposium and contains information on treatment effectiveness of SAP’s/Aluminator combination treatment systems . The following two tables were excerpted from that paper and characterize results from two different mines with flows and chemistry somewhat similar to the E-mine.

Table 1. Coal Run Treatment System Characterization. Design flow; 300 gpm
Sample Point	pH¹	alkalinity²	acidity²	iron³	aluminum³
Raw4	2.8	0	400	50	40
Raw5	3.2	0	220	20	15
Aluminator©	6.2	55	75	18	2
Settling Basin	6.1	30	40	1.5	0.8
SAPS	7.1	65	0	<0.3	<0.2
Final Discharge	7.0	60	0	<0.3	<0.2
1s.u.; 2as mg/L CaCO3; 3total mg/L; 4design values; 5more recent quality developed from additional mine drainage treatment in the watershed

 

Table 2. Metro (M1) and (M2) Treatment System Characterizations
Sample Point	pH¹	alkalinity²	acidity²	iron³	aluminum³
M1 Raw	2.8	0	1,300	270	90
M1 Aluminator©	5.8	90	240	140	20
M2 Raw	2.7	0	1,400	290	110
M2 Aluminator©	5.8	100	410	170	25
1s.u.; 2as mg/L CaCO3; 3total mg/L

The Metro was designed for flows of 30 gpm, but treats flows of up to 200 gpm.

Earlier SAP’s systems were reviewed by Skousen and are found in the appendices of URS’s report

“A 1022 m 2 surface flow wetland was constructed in KY to treat 37 L/min of AMD with a pH of 3.3,acidity of 2280 mg/L as CaCO 3 , Fe of 962 mg/L, Mn of 11 mg/L, and Al of 14 mg/L (Karathanasis and Barton 1997). After construction in 1989, metal concentrations in the effluent were reduced during the first six months of treatment, however, the system failed thereafter due to insufficient wetland area and metal overloading. In 1995, a two-phase renovation project began incorporating the use of an ALD, and a series of anaerobic drains that promote vertical flow through limestone beds overlain by organic compost (much like a SAPS). Results to date indicate a pH of 6.4, slightly net alkaline water (15 mg/L as CaCO 3 ), Fe reduction of 96%, Mn removal of 50%, and Al by 100%.

Kepler and McCleary (1994) reported on initial successes for three SAPS in PA. The Howe Bridge SAPS reduced acidity from 320 mg/L to 93 mg/L as CaCO 3 , and removed 2 mg/L ferric iron. The REM SAPS decreased acidity from 173 to 88 mg/L as CaCO 3 , and exported more ferrous iron than entered. The Schnepp Road SAPS decreased acidity from 84 to 5 mg/L as CaCO 3 , but removed all 19 mg/L ferric iron, with only 1 mg/L ferrous iron exiting the wetland. Kepler and McCleary (1997) reported the use of SAPs in OH, PA, and WV. In all cases, Al in AMD precipitated in their systems. Their drainage design incorporates a flushing system called the 'Aluminator' (Picture 10). This allows for the precipitated Al to be flushed from the pipes thereby maintaining hydraulic conductivity through the limestone and pipes. One SAPS, Buckeye, received 3 L/min of very acid water (pH of 4.0, acidity of 1989 mg/L as CaCO 3 ), Fe of 1005 mg/L, and Al of 41 mg/L. Over a two-year period, the effluent had a pH of 5.9, net acidity concentration of about 1000 mg/L, Fe of 866 mg/L, and <1 mg/L Al. A second site, Greendale, treated a 25-L/min flow, and increased the pH from 2.8 to 6.5, changed the water from a net acid water (925 mg/L as CaCO 3 ) to a net alkaline water (150 mg/L as CaCO 3 ), Fe from 40 to 35 mg/L, and Al from 140 to <1 mg/L.”.(Skousen)

SAPs and Aluminator combinations have been designed for variable flow 4 – 9000 gpm and 5 – 6000 mg/l of acidity with flows of 130 gpm.

SAPs require less residence time than anaerobic wetlands (they can be sized smaller). Iron concentrations are still high as water exits the SAP. Settling pond(s) downstream of the SAP can be used to remove iron and aluminum precipitates

Vertical flow ponds are essentially an imitator of a SAP. Some of the systems reviewed were poorly designed vertical flow ponds. One of the systems reviewed by Demchak et. al, was subsequently dug up, the compost layer 12 years into a system designed with a 15 year design life was found to be intact. The designers of SAP’s have found that systems that were 60% or 100% effective 10 years ago have the same removal capacity if they are maintained properly. Kepler and McCleary have reported on over 100 SAPS systems that they have installed. They say it is critical to use the Aluminator to flush the system where aluminum is present, and to install flow controls to adjust for variable flow.

SETTLING PONDS AND AEROBIC WETLANDS

All of the systems designed to date include settling ponds. The EMSG study includes aerobic and anaerobic wetlands, and URS recommends aerobic wetlands and SRB’s which are essentially anaerobic wetlands. The use of both of these technologies is well documented. Iron precipitates out upon exposure to oxygen. The settling ponds allow a large enough area for the velocities to slow down, the water to oxidize, and a still enough environment (a pond) that will allow the iron to settle to the bottom. While the water is in the settling pond the pH is increased and metals drop out. Settling ponds can be used to remove iron prior to entry into an anaerobic wetland or SAPS, and to remove iron from the effluent of an anaerobic wetland or SAPS. This precipitate needs to be cleaned out on a regular basis. Some work has been done in the past on recycling these metals from the settling ponds, but no information was given in the study about metal recycling. Pilot studies on resource recovery from settling ponds have been undertaken, but the results are not cited. The costs could be used to offset O and M costs.

Aerobic wetlands do a lot of what settling ponds do, they allow the iron to contact with the air and precipitate as metal oxides and metal hydroxides. Settling ponds are better for large concentrations of metals and aerobic wetlands are best for small concentrations of metal. A down-side of aerobic wetlands is that they increase acidity (decrease pH) when the available alkalinity is used up.

Most wetlands have both an aerobic and anaerobic component. The Canadian study stated that wetlands that treat acid mine drainage “will reach toxic levels much sooner [than constructed wetlands that treat sewage waste], rendering their long-term applicability (>100 years) doubtful.”

URS recommends algal mats. Algal mats remove dissolved metals, but pass suspended metals. They require a back-up settling area and do not function in winter when frozen.