Total Dissolved Solids

Total dissolved solids (TDS) are the measure of dissolved material in a liquid. The most common constituents in water are dissolved salts and minerals such as calcium, sodium, sulfate, magnesium, chloride, and potassium. The Environmental Protection Agency (EPA) has established a ‘Secondary Drinking Water Standard’ for TDS of 500 milligrams per liter (mg/L). As a secondary standard, levels in this category are non-enforceable and have no known detrimental health effects, but only an aesthetic effect.

Discover pixabay free images no hotlinking

Aesthetic Effects

Depending on the TDS level, odor, color can be effected and the taste. The taste of water can differ depending on the level of TDS and a personal preference in some instances. A study by the World Health Organization came to the following conclusions on acceptable TDS levels for taste:

TDS Levels (mg/L)Rating
Less than 300Excellent
300 to 600Good
600 to 900Fair
900 to 1,200Poor
Above 1,200Unacceptable


Development of a New Treatment Plant

Cherokee Metropolitan District’s (CMD) Water Reclamation Facility (WRF) was on the drawing table back in the late 1990’s. At that time, the current wastewater treatment process had reached 75% of its capacity. In an effort to meet increasing regulations and limited available land adjacent to the current treatment system, research, design and land acquisition for a new state of the art wastewater treatment process began. Cherokee started the design and research for the WRF based on the Colorado Water Quality Control Division’s (WQCD) issuance of a detailed list of preliminary effluent limitations (PELs) that the treated water would have to meet. The list of PELs received from the WQCD made no mention of a limit for TDS.

Submissions for Approval

PELs where submitted by Cherokee on May 2, 2006 with subsequent approval by the Colorado WQCD on June 15, 2006. Based on the WQCD’s approval of the PELs, site location and design approval documents were prepared. Site approval from the WQCD came on august 25, 2006 with design approval coming in January and February 2007 and June 2008. With the final approvals received, Cherokee authorized construction on June 27, 2008. In August of 2009, Cherokee submitted an application for a discharge permit to the Colorado Department of Public Health and Environment (CDPHE), based upon the approvals received from the WQCD.

Down to the Wire

On March 19, 2010, the CDPHE informed Cherokee that a limit for TDS of 400 mg/L would be included into the discharge permit. At this time, the new plant construction had been well underway with only 3 months left of a 2-year project. With the new plant scheduled to start taking wastewater flows in June 2010, Cherokee requested a revision to the TDS limit. Although the plant was not designed to remove TDS, and despite the previous WQCD’s approved PELs, that did not include a TDS limit, the CDPHE denied the request. 

The CDPHE issued a final discharge permit on May 13, 2010 containing a TDS limit of 400 mg/L to go into effect June 12, 2010. The TDS limit was based upon Colorado Regulation Number 41, ‘Basics Standards for Ground Water’, table Number 4.

Start Up

With the newly imposed TDS limit and the new plant start up only months away, Cherokee had no other alternative than to begin receiving flows in June of 2010. Subsequently, Cherokee set forth a course to achieve compliance.

State Enforcement

In 2014, the district received a Compliance Order on Consent (COC) from the CDPHE, with required actions and timelines. Part of the requirements of the COC is a Control Strategy Plan, which would entail a Feasibility Study outlining the possible alternatives and approaches to the source management and treatment options. In part, the Feasibility Study contains data Cherokee had been gathering to achieve compliance prior to receiving the COC. 

In 2014, the detailed study outlining our options was completed. With the study completed, an Implementation Plan was developed and completed in January of 2015. That plan, included control strategies required in the COC. Because treating water to remove TDS is costly, we are currently exploring the less costly options of source management and control as well as regulatory options.

Source Management & Control

As part of source management and control, we are managing our water supply sources to limit the TDS entering our water system to the extent possible. This includes managing the use of our existing wells, as well as developing new, lower TDS, water from our Sundance well field. We are working closely with our industrial customers on promoting best management practices. We have also restricted the use of home water softeners because these water softeners add TDS to our wastewater, resolution 14-06.

Regulatory Options

We are pursuing regulatory options as well. As part of our renewal application for the WRF discharge permit, we have requested that the WQCD grant a compliance schedule or a temporary variance to the 400 mg/L, TDS limit. These options would suspend further permit violations while we work on a permanent solution. Continuing violation of the TDS limit and the potential fines that could result, are counterproductive to resolving this complicated issue. In 2016, we will also be presenting our case to the state’s Water Quality Control Commission for a sensible change to the groundwater regulations, or a permanent measure that allows a more reasonable TDS limit at the WRF.

Preparing for the Future

Meanwhile, we are preparing additional treatment of our wastewater to remove TDS. Microfiltration (M/F) and Reverse Osmosis (R/O) are the industry standard processes for the removal of TDS TDS is very costly to remove, the R/O process produces a brine waste stream, which in itself, is a challenge for disposal. In order to plan and properly size the TDS removal processes, we continue collecting TDS data, throughout our water and wastewater systems. Construction would have to begin within the next five years in order to meet our compliance schedule.

TDS permit compliance is a very challenging issue. We are doing our best to address it in a sensible way, with you, our customer, and other stakeholders in mind. We will keep you informed as we work toward a solution.