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Total dissolved solids (TDS) are measured by letting liquid samples pass through a filter. Some solids are left on the filter, but dissolved solids go through the filter and remain in the liquid. This liquid is placed into a pre-weighed ceramic crucible (bowl), placed in an oven at 180 degrees Celsius for 1 hour, removed and cooled. Once the crucible has cooled, it is then weighed again. The weights of the crucible are then subtracted to determine the TDS in milligrams per liter (mg/l).
A few processes such as electro dialysis reversal and reverse osmosis can remove TDS (salts) from wastewaters. Reverse osmosis is the most commonly used of these. Reverse osmosis is a process wherein water is forced through a membrane with extremely small pores, or openings of approximately 001 microns.
A process for the removal of suspended, colloidal and particulate matter from liquid, with various pore sizes depending on the desired treatment. Microfiltration has the largest pore sizes, whereas reverse osmosis has the smallest relative to the four types illustrated below:
Microfiltration is a filtration process where contaminated fluid is passed through a special pore-sized membrane, approximately 0.1000 to 0.2000 microns, to remove microorganisms and suspended particles.
Reverse osmosis (R/O) is a filtration process; the membrane (filter) size is approximately 001 microns (5). Because of the filter size being so small, and layer upon layer, pressure must be applied to force the water through these membranes. These membranes collect minerals (salts) on one side. While on the other side, clean water is the end product.
To put the R/O process and membranes into perspective, when you brew coffee, you use a one-layer filter that can have a pore size of 5 to 100 microns. An R/O membrane has much smaller openings, (an understatement), and a number of layers. In addition, while gravity is used when brewing your coffee, pressure is applied to an R/O filter to push the water through the multitude of layers and small pore sizes. See the below diagram.
Cherokee’s current plant is a Biological Wastewater Treatment Facility. In a biological process, ammonia, nitrates, and phosphorus are reduced through aeration and naturally occurring microbiological chemical reactions.
Another aspect of this type of treatment is the removal of total suspended solids, and other undesirable components. This process is completed through coagulation and sedimentation and makes up what is referred to as biomass. Once the biomass has settled it is removed to another part of the treatment process. The current plant was not designed to remove TDS and currently only removes approximately 5%.
Typically, a plants design is submitted to the state regulatory agency. In our case, the plant design was based on preliminary effluent limits (PELs) issued in 2006 by Colorado’s Water Quality Control Division (WQCD). In 2007, the WQCD approved the design for meeting the PELs, which did not include a total dissolved solids (TDS) limit at that time. With that design approval, construction for the plant broke ground in 2008.
On August 21, 2009, Cherokee applied for a state discharge permit, and that was approved on May 13, 2010, only months before receiving flows in June 2010. It was only then that a new requirement was imposed, limiting TDS to only 400 milligrams per liter (mg/l). Unfortunately, the plant had not been designed for TDS removal. If in the designers had been aware of the TDS limit in 2007, they could have designed the plant for treatment of TDS.
Metal plating and powder coating industries are within the district’s boundaries.