By Baseline Water/Wastewater Engineering
Water/wastewater treatment is an ongoing, high priority in Colorado and there’s no shortage of news about it—especially with discharge regulations getting tighter. Remember the controversy about the strict standards for arsenic levels CDOT encountered on its highway improvement project at I-25 and West Alameda? A similar issue happened to RTD on its excavation for an underground bus station at Denver’s Union Station. In both situations, these agencies had difficulty finding options to meet the standards, and they all came with high price tags.
In Denver, compliant dewatering practices are the next 800 lb. gorilla facing developers. If they’re not approached properly, they can break a project. Baseline was recently retained by a prominent, multi-family developer in Denver. This client’s project is a good example of why it’s smart to be proactive instead of reactive when encountering contaminated groundwater at a construction site.
When vertical construction began on the project last spring, groundwater contaminated with elevated levels of selenium entered the basement sump. This problem posed a unique challenge. “Nuisance” water, ranging from 1,500 to 4,000 gallons per day, needed to be treated from 11 parts per billion (ppb) dissolved selenium to below 4.6 ppb prior to being discharged into the local creek’s storm tile. The 4.6 ppb discharge limit is set by the Colorado Department of Public Health and Environment. It’s anticipated future regulations may lower the dissolved selenium to less than 1 ppb, which will be much more expensive to build and maintain than the current requirement.
In addition, nuisance water had to be continually pumped from the basement sump to prevent heaving in the ground floor slab and water intrusion in the elevator shaft.
Baseline Engineering used their expertise in designing municipal adsorptive treatment facilities to successfully pilot, design and construct a five gallon per minute selenium treatment system. This system was constructed within four months of “notice to proceed”. The plan immediately saved $8,000 to $12,000 per month in water hauling fees. It also eliminated the need to lease a temporary storage tank and its property.
The pilot tests were the foundation for the system’s success. They evaluated several adsorptive media technologies under identical operating conditions to see which one performed the best with respect to:
- Selenium Removal Efficiency – What’s the percent removal?
- Media Life – How long do media remove selenium to below 4.6 ppb?
In an effort to reduce pilot costs, Baseline convinced manufacturers to donate their media.
Contaminated groundwater was pumped out of the basement sump through four respective systems ranging from adsorption to anion exchange. Consistent flow rate and empty bed contact time were properly maintained. A flow meter recorded the instantaneous and total flow through each system so that a breakthrough curve could be established. Pressure loss data versus flow rate were also recorded for each system to compare energy consumption.
Baseline Engineering designed and built a full-scale system around the best performing media and combination pre-treatment, longest runtime and highest flow-to-pressure loss ratio. The design consisted of:
- New feed pumps
- Valve tree designed to operate the pressure vessels in a lead/lag and parallel
- Pre-treatment system that significantly improved the removal efficiency and media life
What can real estate developers do to be proactive and save money on groundwater remediation issues like this developer instead of being reactive and lose money? There are three priorities:
- Be aware of potential problems and develop an action plan to solve them.
- Get assistance in developing a pilot test program that evaluates several technologies to determine capital and ongoing operation investments.
- Based on pilot results, construct a full-scale system design to consistently remove target contaminates below the required standards.
For more information and a FREE project assessment, contact firstname.lastname@example.org or (303) 940-9966.