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-1-

September 21, 2015 

Mr. Phillip Flanders 

Engineering & Analysis Division (4303T) 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

Subject: COMMENTS ON NOTES FROM MEETING BETWEEN ERG AND GEA EGI 

Dear Mr. Flanders: 

This letter and the attached document was prepared in response to a request received from 

Ms. Elizabeth Gentile, Environmental Engineer, Eastern Research Group (ERG) dated 

September 9, 2015. Ms. Gentile sent meeting notes that were taken by ERG during a 

meeting that took place on Marh 31, 2014. At this meeting Mr. Istvan Kovacsics of GEA 

EGI Contracting Engineering Co., Ltd. (GEA EGI) provided a technical briefing to a group 

of ERG and EPA attendees. The topic of the briefing was the Circumix Dense Slurry System 

(DSS) ash managment technology. As you know, you were one of the attendees. 

This letter and response is being jointly submitted by NAES Corporation (NAES) and GEA 

EGI. NAES and GEA EGI formed an alliance in 2013 appointing NAES as the exclusive 

provider of the Circumix DSS technology in North America. This submittal is on NAES 

letterhead and some of the comments were prepared by me However, the comments, quotes 

and corrections made to the subjectmatter presented herein were prepared and/or approved by 

Mr. Kovacsics. 

Quite a bit of time has elapsed since the meeting so it is appropriate to review the 

discussions. GEA EGI and NAES Corporation have taken this opportunity to offer 

clairifications and details in the attached marked-up version of the document in an effort to 

make the final text as accurate as possible. There are quite a few clarifications. 

In addition to the contents of the draft notes document, there were some other topics that 

were discussed in the meeting, including the operation principle and controllability of the 

Circumix mixer, possibility of processing FGD gypsum with ash, typical mixer sizes, 

references, history of the technology, special features of pipelines; details of potential US 

DSS projects; major overhaul times, the global distribution of dense slurry and other 

technologies, physical parameters of the solidified end-product; and how to properly 

construct a disposal site (drainage, isolation); covering existing impoundments with dense 

slurry. There were cursory discussions on these topics during the meeting but the draft 

ERG Meeting Notes on Dense Slurry Incorporating GEA EGI & NAES Comments September 21, 2015 

Page 1 of 5 

Notes from Meeting with GEA EGI Contracting/Engineering Co., Ltd. 

March 31, 2014

 On Monday, March 31, 2014, EPA and ERG participated in a meeting with GEA EGI 

Contracting/Engineering Co., Ltd. (GEA EGI) at EPA Headquarters in Washington, D.C. to discuss GEA 

EGI’s Circumix Dense Slurry System (DSS) technology. Participants in the meeting included 

representatives from EPA, ERG, and GEA EGI, as listed in Table 1. 

Table 1. List of Participants

Name Organization

István Kovacsics GEA EGI

Ron Jordan EPA

Phillip Flanders EPA

Jezebele Alicea EPA

Deb Bartram ERG (contractor to EPA)

Elizabeth Gentile ERG (contractor to EPA)

 Mr. Kovacsics visited EPA to give a presentation on their DSS ash handling technology. The 

presentation, company brochures, and other supporting documents provided by Mr. Kovacsics following 

the meeting are included as attachments to these notes and listed in Table 2 below. Mr. Kovacsics also

provided copies of the public comments submitted on this rule and the CCR rule (EPA-HQ-RCRA-2013-

0209), which are available in the dockets. 

Table 2. List of Attachments

DCN Title

SE05936A1 GEA Circumix® Dense Slurry Technology Brochure

SE05936A2 NAES Circumix® Dense Slurry System (DSS) for Ash and Waste Water Management 

and How it Addresses Proposed ELG and CCR Rule Change 

SE05936A3 NAES Corporation DSS Presentation

SE05936A4 NAES Statement of Qualifications

SE05936A5 NAES Corporation Brochure

SE05936A6 Year Two Follow-Up Investigation Report: Soil and Water Quality Evaluation St. 

Mary’s Shoals Park Roadway Improvements 

SE05936A7 GEA EGI DSS Technology Photos

 Mr. Kovacsics started with an introduction of the DSS technology, stating that if certain 

conditions are met, mixing water with fly ash results in a product that solidifies naturally (due to the 

compounds, chemical reactions and mineral structures that are formed). These reactions involve 

reactive compounds such as CaO, Al2O3, MgO, SiO2 and are affected by slurry conditions including the 

quantity of water, ash particle size, chemistry of the water, availability of reactive grain surfaces and 

slurry homogeneity. For each installation, GEA EGI evaluates a diverse set of inputs to determine if the 

DSS technology is an applicable ash handling technology for a specific plant. GEA EGI has been 

experiencing a high level of interest in the system from the industry. 
ERG Meeting Notes on Dense Slurry Incorporating GEA EGI & NAES Comments September 21, 2015 

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 The system is designed for coal-fired generating units and one system is operating at a cokefired plant; however, it is not an available technology for oil-fired generating units (mainly due to the 

negligible amount of ash generated). Extensive tests and working references have shown that the 

technology can work with practically all coals from lignite and sub-bituminous coals to bituminous coals 

and anthracite. Mr. Kovacsics stated that pulverized coal boilers are preferred for the DSS technology

because of the high furnace temperature. Fly ash and bottom ash from circulating fluidized bed boilers

are also good candidates for the system because plants often add lime to the fluidized bed, resulting in 

high calcium oxide content of the ash, and the bottom ash is very reactive (like the JEA Northside plant 

in Jacksonville, FL)). 

 In order for an ash slurry system to function efficiently, it is important to identify the optimal 

level of water. If the slurry is too thick, energy consumption for pumping can be very high and capital 

costs can increase significantly. If the slurry is too thin, the excess water can inhibit solidification 

reactions, energy costs increase due to higher pumping volume and pipe abrasion can increase due to 

larger particles rolling or sliding along the bottom of the pipe. The solids to water ratio in a traditional 

wet sluicing system ranges from 1:10 to 1:15 which is far from the optimum. In a DSS, the solids to water 

ratio typically ranges from 1:1 to 1.5:1 (between 50% and 60% solids). 

 Typically, fly ash is only considered in the hydraulic calculations for the design of the DSS. 

Bottom ash is too coarse to affect the viscosity of the slurry. When designing the system, GEA EGI 

focuses on the extremes of different fuel types or coal blends that are used at the plant to calculate the 

optimum solids-to-water ratio for the slurry, pipe diameters, mixer dimensions and other parameters of

the DSS system. The plant can make adjustments to this solids-to-water ratio when mixing the slurry if

necessary to adapt to changing coal quality. Typically, a plant does not need to adjust the solids to water 

ratio day to day unless a very different type of coal is being burned. In terms of load control, the 

Circumix system is flexible since it uses variable frequency drives: the typical control range of a mixer’s 

output is 40-110%. 

 The mixer developed for the system makes the mixing of the dense slurry more efficient. In a 

pug mill-based technology (which is commonly used), the mixing is typically done in two phases: the 

pugmill (which is a screw with no paddles) first wets the dry fly ash and then drops it into to a larger, 

agitated mixing tank (second phase) where the mixing and adjustment of solids to water ratio takes 

place. This is where paddles are typically used for agitation. Pug mill technology suffers from the 

following drawbacks: 

• Solidified deposits are easily formed in the pug mills. These deposits need to be frequently 

removed by hard manual labor. This means cost and downtime. 

• Wetting in the pug mill is commonly inadequate. Large lumps are usually formed which are 

either dissolved in the mixing process later or are transferred as-is to the disposal site. 

• There are dead zones in the mixing tank which cannot be reached by the paddles. This is where 

settling occurs and deposits usually form. 

• Paddle mixing results in an inhomogeneous mixture which adversely affects end-product 

quality. 

• The wear/abrasion of the agitating paddles is high, leading to increased maintenance costs and 

downtimes. 

In contrast, the Circumix DSS mixer implements a four-stage mixing procedure: Mixing takes place in: 
ERG Meeting Notes on Dense Slurry Incorporating GEA EGI & NAES Comments September 21, 2015 

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• the premixer head where the dry ash is wetted by high velocity jets of recirculated slurry, 

• in the two recirculation pumps where the impellers of the pumps intensively mix the slurry, 

• in the mixer tank where the entire contents of the tank is kept in constant swirling motion by 

the tank recirculation and, 

• in the transport pipes where the flow is constantly kept in the turbulent range. 

This very intensive mixing process results in a highly homogenous mixture and thus in very good endproduct quality. 

Mr. Jordan asked if bottom ash that is currently wet sluiced could be transported by the DSS. Mr. 

Kovacsics stated that bottom ash can be and in most of the reference plants is processed and 

transported with DSS. As the dense slurry requires a closely controlled s/w ratio, some of the water of 

the sluiced bottom ash, however, must be removed by thickeners or other dewatering devices such as 

hydrocyclones or settling tanks. The partially dewatered bottom ash slurry can then be introduced to 

the Circumix mixer and processed together with fly ash. 

When processing bottom ash, it is important to keep its large and abrasive particles in suspension to 

avoid settling and thus to minimize power consumption and transport pipe abrasion. The viscosity of the 

dense slurry is adjusted exactly so that it would be sufficient to keep these particles suspended but not 

too high to require too high pumping power. In order to process and transport bottom ash, the largest 

particles (variable but about one inch diameter) must be crushed or removed prior to mixing with the 

slurry. 

Mr. Kovacsics stated that unlike bottom ash, gypsum helps with adjusting the viscosity of the dense 

slurry because it is a fine powder. 

Mr. Kovacsics reiterated that all of these systems are custom built. Additionally, he stated that the 

configuration of the ash handling system depends on the ash quantities and the philosophy of 

availability for the plant. Usually there must be some redundancy to maintain high availability. If there is 

one unit then 2 mixers and transport systems are usually employed. In case of multiple units, the most

common configuration is 3 mixers per two units (50% redundancy) but other configurations are also 

possible. Normally each mixer is connected to a dedicated pump train and their own transport pipeline.

However there are backup mixers (100% or 50% redundancy typically) with their own pump trains. 

There are crossover connections that make it possible to connect any pump train to any mixer and any 

pump train to any pipeline in order to increase operational availability. 

In order to pump the slurry to the final destination, GEA EGI prefers to use centrifugal pumps over the 

use of piston diaphragm pumps (PDP) because centrifugal pumps require less maintenance, are less 

expensive and, more importantly, are highly tolerant of larger particles (as found in bottom ash). Mr.

Kovacsics stated that the technically feasible conveying distance for the DSS technology ranges from 15 

to 20 kilometers, although that distance can be influenced by elevation differences. A further benefit of 

DSS is that owing to the optimum viscosity, standard steel pipes can be used for slurry transport, no 

expensive ceramic lining or other anti-abrasion measure is necessary. 

Mr. Kovacsics explained that in the conversion of a traditional wet sluicing system, DSS can be 

integrated in any type of existing ash collection and storage system, regardless of whether the bottom

ash system is dry or wet. Some of the existing infrastructure can be retained and utilized, the most 

important of which is the existing bottom ash system which typically does not require modification. This 

is in contrast to a dry ash conversion which requires costly replacement/modification of the boiler 

bottom and/or bottom ash collection and transport systems. In a DSS conversion project the existing 

system remains the same up to and including the fly ash silos. Only a new mixer, transport pumps and 
ERG Meeting Notes on Dense Slurry Incorporating GEA EGI & NAES Comments September 21, 2015 

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transport pipelines (and in the case of bottom ash processing) partial dewatering capabilities need to be 

added. 

Mr. Kovacsics stated that the total EPC cost of these systems can range from anywhere to $5 million to

$60 million dollars, depending on plant size, number of units, redundancy requirements and what CCRs 

need to be processed, excluding the costs related to the landfill. It is important that as working 

reference plants confirm, DSS can process various CCRs including fly ash, economizer ash, bottoms ash,

FGD gypsum and FGD waste water or other plant waste waters in one system. 

The main maintenance item associated with the system is with the pumps; however, the standard 

centrifugal slurry pumps usually employed are easy to repair. If the pumps are running 24 hours per day, 

the impellers and volute liners need to be replaced every 6,000 to 15,000 operational hours depending 

upon ash properties, such as SiO2 content. The parts of the system that are exposed to heavy abrasion , 

such as bottom ash slurry inlets and the mixer internals are lined with ceramic chips. Overall 

maintenance period is long: major overhauls are needed every 5-8 years. As to normal operation, an 

operator must inspect the systems and the entire length of pipe at least once a day for any leaks or 

other faults. 

The buffer capacity of the silos which are part of the system enables uninterrupted plant operation 

when the ash processing system is shut down for short repairs or maintenance. The silos for the system 

are usually designed for 8-24 hours of capacity. Mr. Kovacsics stated that this is enough capacity 

because any maintenance issue should be able to be resolved within 8 hours provided the plant has the 

stocked spare parts required for the maintenance. 

Makeup water for the system is required; however, it can come from other wastewaters at the plant 

(e.g., bottom ash transport water or wet cooling tower blowdown). Plants can also use FGD wastewater; 

GEA EGI has confirmed that chlorides in the FGD wastewater are not an issue in the system. GEA EGI 

confirmed that there are four plants (three in Romania, one in Hungary) using FGD wastewater as 

makeup water for the DSS. Flushing water is also required when the system is stopped to flush the slurry 

pipelines to prevent solidification of the slurry and blocking of the system. In normal operation such

flushing is needed typically every 1-2 weeks, thus the quantity of flushing water is low, about 0.2% of 

the weekly slurry quantity. 

The resulting ash by-product is a solid that cures in the landfill. The compressional strength of the cured 

deposit typically ranges from about 200 psi to over 2500 psi. The hydraulic conductivity is in the range of 

1x10-5 to 1x10-10. Hydraulic conductivity and compressional strength both improve as curing progresses 

and the ash by-product does not generate any dust. The resulting by-product would not typically be 

marketed. 

At the slurry disposal site, plants typically split the disposal area into cells and dikes are 

constructed to separate and surround the cells. Using this approach, plants can discharge the dense 

slurry into one cell, keep another cell in reserve, and work on the preparation of additional cells. 

Discharged slurry spreads out and cures in place without the need to spread it. Because of the 

composition of the material, the landfill can be built vertically in tiers. Mr. Kovacsics stated that water 

does not need to be added to the landfill for dust suppression, nor it is necessary to scrape or spread the 

ash by-product. Two operators are required at the landfill only intermittently (every 1-2 weeks) to 

switch the outflow point. Other on-site works include preparing cells and moving the slurry pipelines 

when needed. 

The dense slurry is a roughly 1:1 ash-water mixture. Approximately 5 to 10 percent of the water is 

excess water that becomes a thin film on top of the ash by-product at the landfill. This water either 

evaporates or slowly seeps in and captured by the lower deposit layers. If any excess water remains it is 
ERG Meeting Notes on Dense Slurry Incorporating GEA EGI & NAES Comments September 21, 2015 

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collected along with leachate. This water and any precipitation that is captured are sent back to the 

system for slurry make-up water. GEA EGI confirmed that any leachate associated with the system that 

can be collected and reused as makeup water to achieve zero discharge. The dikes built around the 

landfill are typically 3 meters tall and are typically built using ash by-product. Mr. Kovacsics stated that 

there aren’t any run-on issues and that any excess water from precipitation not absorbed by the curing

product can be recycled to the process together with leachate. 

Since the dense slurry cures to a hard product, it is suitable for capping existing impoundments. 

The DSS may not be ideal in geographic locations that have cold climates (10-15 ̊F) for extended periods 

of time (e.g., Russia). If the slurry freezes it will not harden and set. However, the curing process is an 

exothermic reaction. Therefore, it may be OK in locations that have short periods of freezing weather. 

Plants could also put slurry pipes underground to avoid the freezing of flush water or install trace 

heating.