Sep 07, 2006

Reliable, Repeatable Results

Sampling in the water and wastewater industry is a very important process that requires both labor and proper experience. Although it is simple, specific steps have been included in the regulations to maintain a process that helps the industry achieve reproducible and accurate results, and meet requirements. Sampling is considered part of the daily work cycle, which also includes other processes such as monitoring and sample analysis.

Measured parameters

Total organic carbon (TOC) is one of the parameters measured by the water and wastewater industry to ensure proper treatment and continuous agreement with the regulations. In addition, other parameters are measured, and standards are being implemented as the regulations call for them, for example, measuring nutrients, such as total nitrogen (TN) and total phosphorus (TP), which lead to eutrophication in the environment. The sampling method used to measure such parameters is very important, followed by the treatment method used and the analysis. Such parameters can be measured using a laboratory or online instrument (figures 1A and 1B).

Sampling for analysis of TOC, TN or TP can be performed manually or automatically; however, both ways should be available at a plant in order to accommodate the required method and the accessibility to sampling, i.e. remote versus onsite sampling locations.

Manual sampling & analysis

Manual sampling requires following specific steps and using specific tools to ensure the sampling is performed in the same manner every time, and to ensure the sampling is performed from the same area. Manual sampling is usually followed by sample treatment and analysis, or followed directly by analysis.

To ensure the continuous accuracy and reproducibility that was initiated during the sampling process, the samples must be analyzed in the same manner. This can be done by using an autosampler, which ensures comparable treatment of the samples.

Several autosamplers exist on the market—for example, the ASI-V and OCT-1 autosamplers from Shimadzu Scientific Instruments (figures 2A and 2B). Autosamplers provide customers with the flexibility to use the most appropriate sampling technique—one that will meet their sample’s volume and characteristics, number of samples to be analyzed, required analysis and elimination of background contamination. Autosamplers are aimed toward laboratories performing one-time sampling to continuous 24/7 autosampling. Table 1 presents some features that should be available in an autosampler.

With autosamplers, turnaround time is minimized, and consistency in analysis is achieved. The use of an autosampler also helps to create multiple calibration curves of multiple points from a single standard, which in turn eliminates the human error involved in preparation of standards. In addition, using an autosampler eliminates manual injections, which may be influenced by the technique used.

Automatic sampling/pretreatment systems

Sampling is a long, tedious process that requires both labor and time. To make the process shorter, easier and more practical, and to eliminate traveling to remote areas for sampling, automatic sampling systems have been designed to sample remote and onsite areas automatically, without any human interference. These systems have been built to communicate with instruments used in the water and wastewater industries to allow for remote operation. On the market, there are several automatic sampling products of different shapes and sizes that not only automatically take samples, but also pretreat the samples so they are ready for analysis. Automatic sampling and pretreatment of the samples provide a sampling tool that is consistent and reliable in producing acceptable and reportable results.

Choosing the correct system

Choosing the correct sampling system is an important step in achieving maximum efficiency. Streams, pipelines, rivers and other bodies of water have different characteristics, and it is important to consider these characteristics (e.g. flow rate, presence of particulates, temperature, etc.) when choosing the appropriate sampling/pretreatment sampling system. Choosing the right system ensures consistency in sampling and treatment, which ensures accurate and reproducible results.

Automatic samplers must be able to accommodate the continuous change in the characteristics of the different bodies of water, in order to ensure that there is continuous, smooth operation of the utility, without interference from characteristics such as particulates, which can cause clogging and lead to inaccurate results.

Various automatic samplers and pretreatment units are used in the water and wastewater industry. Such systems can be divided into two main categories: those designed for clean stream systems (Figure 3A) and those designed for particulate stream systems (figures 3B, 3C and 3D). Using an automatic sampling system that can accommodate both clean and particulate streams at the same time (figures 3B, 3C and 3D) is very important, as it reduces capital cost as well as downtime from trying to replace sampling systems to accommodate change in the bodies of water being sampled.

In Figure 3B, the sampling/pretreatment unit uses in-house air to prevent adhesion of contaminants, slime and sludge. In figures 3C and 3D, the units utilize a strainer/homogenizer technology that pulverizes the suspended particles and forms a homogenous solution. The suspended solids pretreatment units reduce problems caused by flow blockage and frequency of maintenance, which is considered one of the main problems in the online analysis industry. Using such sampling/pretreatment systems, therefore, eliminates the use of expensive filters and as a result, reduces cost. The autosampler/pretreatment units are equipped to obtain the minimum sample volume necessary for analysis, which in turn reduces problems from suspended particles as well as reduces problems caused by slime and algae. In addition, the whole system is continuously rinsed with water, and the included strainers are backwashed to eliminate contamination and buildup.

About the author

Mohamed Abouleish, Ph.D., is TOC product coordinator for Shimadzu Scientific Instruments. He can be reached at 410/381-1227, ext. 1343 or by e-mail at [email protected]. Robert Clifford, Ph.D., is TOC Product Manager for Shimadzu Scientific Instruments. He can be reached at 410/381-1227, ext. 1375 or by e-mail at [email protected].