TDS and water quality go hand in hand
Carwash water reclamation systems are growing in popularity for a variety of reasons, including costs associated with tap fees, as well as on-going water and sewer bills.
However, one water quality parameter that is often over-looked in designing and operating a water reclamation system is total dissolved solids or TDS.
It is not possible to visually distinguish between water with high dissolved solids and low dissolved solids.
Therefore, it is important to have an understanding of the factors that affect the TDS concentration in the recycled water in order to control the TDS, and ultimately the quality of wash.
- Source water;
- Softener salt;
- Chemicals used in the wash process; and
- Car residue.
All source water, whether it is supplied by a municipality or comes from a private well, will contain TDS. Such water will typically contain sodium, calcium, magnesium, chloride, sulfate, etc.
The salt added to the system via the softener regeneration process should not enter the system. That is, the brine that is produced during the regeneration process will be extremely high in TDS and should be discharged directly to the sewer.
It should be noted that the softening process does not change the TDS concentration of the water. A salt softener simply exchanges calcium, magnesium, and iron dissolved solids for sodium, which is also a dissolved solid.
Clearly, the removal of hardness is an extremely important step in maintaining the wash water quality. However, with respect to a TDS evaluation, the softening process does not effectively change the TDS concentration.
The two other sources of TDS entering the system are the chemicals that are used and the residue that is removed from the car in the wash process.
The residue that is removed from the car can be a significant source of TDS during the winter months, if the wash is located in a climate that receives a significant amount of snowfall (i.e., from road salt).
How does TDS build up?
TDS leaves the system only in the system discharge. It should be noted that TDS does not exit the system in the evaporation and dragout stream.
There is no TDS in the evaporated water and there is very little TDS in the water that is carried out on the car exterior.
Based on this TDS balance evaluation, as illustrated in Figure 1, it is clear why 100 percent reclaim systems are not possible.
A 100 percent reclaim system would have no system discharge and no way for TDS to exit the system. For that case, the TDS would eventually build up to an unmanageable level.
There is often only one choice of source water. However, if there is another choice (e.g., well water vs. municipal supply), the TDS of the source water would be one factor in making a choice (in addition to such factors as hardness, cost, etc.).
The TDS sourced from the wash chemicals cannot typically be controlled. It is presumed that the chemicals have been carefully selected to produce the best quality wash for the minimum cost.
Similarly, the TDS removed from the car exterior is fixed. That value will vary seasonally in areas where significant snowfall occurs.
Therefore, the primary variable under the carwash operator’s control is the system discharge. The greater the system discharge, the lower the TDS concentration will be in the reclaim water.
The smaller the system discharge, the greater will be the TDS concentration in the reclaim water.
Thus, the system discharge must be balanced in order to maximize the amount of water that is reclaimed, while maintaining the TDS concentration at acceptable levels.
The system discharge rate can be calculated by conducting a system TDS and flow balance.
However, it should be recognized that such calculations are subject to the uncertainties that are associated with the system variables.
Table 1 is a sample spreadsheet, which shows the variables that affect the reclaim water TDS concentration. The usefulness of these calculations lies primarily in the ability to see what impact the system variables have on the reclaim water TDS concentration.
For example, what effect would changing the source water TDS have on the required system discharge rate?
What would be the required change in the system discharge rate during the winter months (i.e., when road salt is present on the car exterior)?
The sample data shown in Table 1 indicates that one would need to use approximately 10 gallons of fresh water makeup per car in order to achieve a reclaim water TDS concentration of 1460 mg/L.
Or, if the target TDS concentration was 1000 mg/L, approximately 20 gallons of fresh water makeup would be needed per car.
To summarize, calculations can be completed to estimate water consumption and wastewater discharge volumes, which would be required to maintain the TDS level in the reclaim water at the desired level.
These calculations can be completed before the carwash is opened, if the following information is available:
- Desired TDS level;
- Source water TDS concentration;
- RO volume applied per car; and
- RO system efficiency.
As shown in Table 1, typical values for the evaporation and carryout, as well as the TDS sourced from the chemicals and car exterior can be assumed.
Summary and recommendations
It is recommended that carwash owners and operators carefully consider all variables when planning a carwash with a water reclaim system.
Estimates of the fresh water makeup rate and system discharge rate can be made by conducting water and TDS balances.
Finally, once a carwash is operational, it is recommended that the system TDS concentration be monitored on a periodic basis.
This can be done by sampling the reclaim water and sending the sample to a laboratory for TDS analysis, (typical cost for TDS analysis is around $10.00).
Alternatively, hand-held TDS meters can be purchased for less than $100.00.
Regardless of the method that is used, it is recommended that TDS be monitored on a periodic basis (e.g., weekly) and system adjustments be made to maintain the reclaim system water quality at the desired level.
Max Shepard is the president of Shepard Engineering, Inc., the chemical engineer for VERwater Environmental, LLC and has over 25 years of experience in the wastewater consulting business. He is a licensed professional engineer in five states. Max can be contacted at email@example.com.