Last month we reviewed the importance of monitoring advanced onsite treatment systems to make sure they are performing as intended.
The key to evaluating performance fairly is to sample the effluent properly. To obtain a uniform sample, you need to take it from the proper location, using the right equipment, at the right time.
The proper location has a couple of dimensions. The first step is to take the sample from the portion of the system that needs to be evaluated. If you need to know the quality of effluent that is going to the soil dispersal system, you need to sample the effluent as it is heading to the soil.
Sampling strategies
The key here is to collect an average or uniform sample of the wastewater. If there is a pump tank following the pretreatment component, this is where you should take the sample. But be aware that sampling here sometimes can be tricky.
Solids that have washed into the tank can create a sample that is dirtier than the water will be when it leaves the tank through the pump. To solve this problem, you can use two strategies.
The first is to use an Imhoff cone to help settle the solids from a sample collected within the tank. By allowing the heavy solids to sink in the cone for 15 minutes and drawing them off through the bottom, you can standardize the samples.
Second, and more effective, you can add a sampling valve on the pump line. This allows you to obtain a sample of the effluent that is moving to the soil absorption system through the pump. The valve should be located in the riser for easy access. Turn the pump on and open the valve to fill an Imhoff cone with effluent. Remember to wear gloves and eyewear.
If there is no pump in the sampling location, you may need to build a sampling port. This can be as simple as a 6- or 8-inch cross with four to six inches of sealed pipe below the flow line and a capped pipe extending up to a point where you can access it.
When sampling, remove the effluent that is standing in the pipe, checking the accumulation of solids in the pipe to see how well the solids have been removed in the up-gradient devices. After removing the effluent, place a sampling bottle in the pipe to collect a sample as the effluent flows out of the tank or filter device.
Protecting integrity
Choose a pipe size that will accommodate the sample bottle sizes needed to measure what you are sampling for. When sampling, use a method that mixes the volumes for all the samples you will be taking. If you need 100 ml for BOD5 and 100 ml for TSS and 100 ml for fecal coliform, take a 300- to 500-ml sample and split it into separate containers. This assures that all samples will read the same level of treatment.
Also, be careful not to change the wastewater characteristics. For example, when sampling the temperature, don’t leave the sample sitting in the sun. For some parameters (notably nitrogen and bacteria), the sample needs to be kept cool until it can be analyzed. A good cooler with ice is a short-term solution for this in the field.
Recognize that, even when on ice, many samples need to be delivered to the lab within certain timeframes for the analyses to be accurate. Always sample from cleanest to dirtiest locations in the system to avoid the potential of samples becoming contaminated.
You do need to clean your equipment thoroughly when moving from site to site to avoid poor results in the later samples. When cleaning equipment with chlorine (bleach), make sure the sample containers are well rinsed before reusing. There is nothing worse than showing good bacteria performance and then realizing it is because the bottles were not adequately cleaned and rinsed.
Proper interpretation
Many times a sample needs to be taken from the septic tank. The septic tank is a good place to sample because it has an average of 18 to 36 hours of retention time. The best place to check is in the clear zone below the outlet. This will give a picture of how the system is operating.
Use a small pump and tubing to sample, or use a sampling bucket to take a sample of the clean zone accessed through the outlet sanitary tee. A sampling bucket is a device that lets you sample the tank at a specific depth.
One such device is the Sludge Judge. This is a clear plastic tube with a ball that seals the end. By dipping it into the tank, you can pull a profile of the tank contents. Then place this into an Imhoff cone that can be split into the required samples.
Another option is a capped container that can be opened at the desired depth. This is mounted on a rod and lowered into the tank. A spring holds the container lid closed; when you pull on a cord, the vessel opens, drawing the sample.
Keeping records
Be sure to take the sample from a consistent depth in the tank so you can enter this on the sampling record. This written record will be a key part of the monitoring data.
Any lab that processes your samples will require the chain of custody to be completed. This is a complete record of the events of the sample and how it was transported to the lab. Without this record, the reliability of the results can be questioned.
This form should come from the lab, and they will ask for the expected values. For a residential septic tank, the BOD5 should be in the 130- to 200-mg/l range. For a restaurant, the BOD5 can be much higher — in the 1,100-mg/l range.
TSS values from a residential system can vary, but a properly operating tank should be in the 30- to 65-mg/l range. Fresh wastewater coming from a home will be higher than the values listed here. Also, the form will require the temperature at the time of sampling, so have your thermometer handy.
Bacteria samples are the easiest to contaminate. Be careful to keep sources of bacteria out of the sample container. This includes your equipment and your fingers. If you receive results with an unusually high value, typically the first step is to resample.
This just highlights that you need to take care in the sampling procedures. Many times, these samples are collected in sterile bags supplied from the lab, so be careful to maintain the sterile environments in the containers. Tempera-ture is critical, so it’s important to get samples on ice quickly.
The final piece is interpreting the results from the lab. Your regulations should help with this by setting or specifying the required performance. Check with your local regulatory authority to see how the system will be rated if it does not meet these standards. If the system is not performing, additional study and testing may be required to get the system operating again.
