COAL TAR RECOVERY: LESSONS LEARNED AT A FORMER MANUFACTURED GAS PLANT SITE

Transcription

1 COAL TAR RECOVERY: LESSONS LEARNED AT A FORMER MANUFACTURED GAS PLANT SITE By Joseph E. Higgins, P.E., L.S.P., Michael Lotti, L.S.P. (Innovative Engineering Solutions, Inc. - Walpole, Massachusetts) Elizabeth Greene, L.S.P. (National Grid - Westborough, Massachusetts) Background Former MGP site with coal tar released. Silty clay perching layer encountered at a depth of about 25 feet Groundwater present at a depth of 14 feet Regional groundwater present at a depth of 45 feet Contamination coincident with the silty clay layer Coal tar DNAPL observed at thicknesses greater than 8 feet above the silty clay layer Estimated 100,000 gallons downward movement of the DNAPL limited by perching layer.

2 Extent of Contamination Up to 8 feet of DNAPL has been measured on top of the perched layer. The perched water table is the result of a silty clay layer approximately 150 by 100 feet in area. Profile

3 Remedial Strategy Recover NAPL Initial recovery efforts 9 peristaltic pumps operating on timers 8,000 gallons of fluid (primarily NAPL) were recovered in 1 year Dewater perching layer Pump/fluid recovery test conducted 600,000 gallons of fluid on the perching layer Recharge of precipitation contributes 1,000,000 gallons of water to the layer annually. Recovery system expanded Initial System

4 Expanded System Designed to remove recoverable NAPL and water from perching layer and collect annual recharge volume. Fluids recovered from 20 recovery wells using peristaltic and pneumatic pumps Aboveground separation of fluids NAPL transferred to an on-site storage tank Water treated and discharged to the POTW Bag filters to remove particulates Granular activated carbon (GAC) to remove organics. Expanded System

5 Manifold Problems Encountered Compatibility with System Components Pump performance Cyanide in Groundwater

6 System Component Compatibility System components Steel Separator (epoxy coated) HDPE for underground piping, NAPL storage tanks PVC for separator coalescing media, aboveground piping System component compatibility with coal tar considered PVC monitoring wells and recovery wells Some 10 years old No signs of deterioration Separator

7 Compatibility Upgraded System Replaced PVC coalescing media with stainless steel Replaced HDPE NAPL storage tanks with stainless steel Replace aboveground piping in manifold with stainless steel and brass Monitor other system components Installed vapor phase treatment on tanks and separator

8 Upgraded System Upgraded System Before After

9 Pump Performance Peristaltic pumps used for initial system Suction limitations Compatibility with coal tar Low recovery Pneumatic pumps used for expanded system Initially used off the shelf bottom loading pumps Float/controller required frequent maintenance Designed and constructed bottom loading pump Operates on a timer Control panel at well head Easier to adjust recovery rates Peristaltic Setup

10 Pneumatic Pump Control Panel Cyanide in Groundwater Cyanide concentrations slowly increased over time, resulting in excessive GAC replacement POTW cyanide discharge limit of 630 ug/l 5,000 pounds of GAC were consumed after treating about 200,000 gallons of water). Conducted column tests using activated carbons, organoclays, and resins Evaluated effectiveness in removing cyanide and oil and grease Oil and grease a primary component in the influent POTW limit of 50 mg/l; the use of one media would be preferable over two media

11 Cyanide Removal Use the existing GAC filters for oil and grease removal Use an anionic exchange resin for cyanide removal System modified 4,000 pounds of GAC 160 pounds of resin Modified system has treated more than 500,000 gallons of water without requiring media replacement. System Costs Capital Costs OMM Costs Total Costs Tar Recovered (Gallons) Water Treated (Gallons) Initial System Expanded System $150,000 $165,000 $315,000 4,000 4,000 $400,000 $950,000 $1,540,000 47,156 2,312,686 Component Upgrades $140,000 Cyanide Issue $50,000 Total $740,000 $1,115,000 $1,855,000 51,156 2,316,686

12 Fluid Recovery System Costs Capital Costs OMM Costs Per Gallon of Tar Recovered Per Gallon of Water Treated Per Gallon of Tar Recovered Per Gallon of Water Treated Initial System $78.75 $78.75 $41.25 $36.67 Expanded System $32.66 $0.67 $20.23 $0.41

13 Summary Points Component compatibility is critical After 4 years of operation, PVC and HDPE piping still intact Many types of PVC, some hard, some soft, PVC coalescing media may not have been resistant to the tar, PVC pipe may be PVC pipe typically weakens at glued joint, not at pipe HDPE can get soft with tar May not have structural support needed for a tank May be adequate as small diameter buried pipe PVC well screens may have swelled and reduced potential for tar recovery Summary Points (cont d) Tar removal less complicated using a total fluids system with aboveground separation Tar viscosity will affect pump selection Tar viscosity can change over time Evaluate discharge options for the water before designing treatment system System operation continues to reduce fluid on the layer After 4 years of operation, tar thickness reduced by more than 7 feet, to less than 1 foot Currently evaluating in-situ methods to enhance tar recovery ph adjustments, surfactants