Water soluble solids

Solids

Solids in water refer to the suspended or dissolved matter in drinking, surface and brackish waters as well as domestic and industrial wastewaters. Solids may affect water and wastewater quality in a number of ways. Waters with high dissolved solids are generally of lower quality and may cause adverse physiological responses in the consumer, so a maximum of 500 mg/l of solids is desirable for drinking water. Highly mineralized waters are unsuitable for many industrial applications. Waters with high suspended solids may be unsatisfactory for bathing and other purposes. Solids analysis is important for controlling water and wastewater treatment processes and assessing compliance with regulatory requirements.

The term

Fixed solids – total solids, suspended solids or dissolved solids, which remain in a sample after combustion for a specified time at a specified temperature. ] The determination of fixed and volatile solids does not distinguish exactly between inorganic and organic matter, since some inorganic compounds can be destroyed during combustion. Organic matter can be best determined by measuring total organic carbon (Section 5310), BOD (Section 5210), and COD (Section 5220) methods. [

Settlement solids – are the substances in a sample that separate from suspension and settle out over a specified period of time. Depending on the method used (e.g., 2540F.3b), they may include floating matter.

Total dissolved solids (TDS) – are the fraction of total solids in a water sample that pass through a filter with a nominal pore size of 2,0µm (or smaller) under specified conditions.

Total solids – are the substances that remain in the sample container after evaporation and oven drying at a specified temperature. Total solids include both suspended solids and total dissolved solids that are removed by physical filtration. Whether solids are filtered as “suspended” or “dissolved” depends largely on the thickness, area, pore size, porosity, and type of filter media, as well as the physical nature, particle size, and amount of solids filtered.

Total Suspended Solids (TSS) – The portion of the total solids in a water sample that remains on the filter. Note: Some clays and colloids will pass through a 2-micron filter.

Volatile Solids – The total solids, suspended or dissolved, that are removed and lost from a sample after combustion for a specified time and temperature.] The determination of fixed and volatile solids does not distinguish exactly between inorganic and organic matter, since some inorganic compounds can be destroyed during combustion. Determination of organic matter can be better determined by the total organic carbon (Section 5310), BOD (Section 5210), and COD (Section 5220) methods. [

Sources of Error and Variability

Sampling and measuring two- or three-phase samples can introduce significant errors. Maintain sample uniformity during transfer and carefully ensure sample integrity. If a portion of a sample is added to the container, take this into account when evaluating and reporting results. During drying, some samples form a crust that prevents water evaporation. Special measures are required to control this.

Special care is required for viscous samples, which may absorb air during mixing and make it difficult to measure accurate volumes. Also, special care is required for samples with high levels of dissolved solids, which are difficult to wash completely during filtration.

Since total dissolved solids (TDS) and total suspended solids (TSS) are different fractions of a sample being analyzed, the instrumentation methods and washing techniques used may affect TSS and/or TDS results. To avoid this, be careful not to count the fraction of TDS that is left on the unwashed edges in the area below the funnel filter as the “TSS” weight. If this is suspected, check the filter seal and/or perform additional washing.

If using a pipette to measure and transfer the sample, place the pipette tip in the center (depth and width) of the mixing vessel. If using a magnetic stirrer and stir bar, adjust the speed to cut off larger particles and mix the sample until a more uniform particle size is achieved. Then pipette the sample from the vertical half of the depth and center of the vessel. Centrifugal force can separate particles of different sizes and densities, reducing the accuracy of sampling. Avoid using a magnetic stirrer with samples containing magnetic particles. When using a graduated cylinder, transfer samples to the cylinder immediately after shaking or stirring to prevent sedimentation of sample solids.

The drying temperature, heating time, and sample matrix can affect weight loss due to evaporation of organic matter, mechanically trapped water, water of crystallization, and gases resulting from chemical decomposition by heat. They can also affect weight gain due to oxidation. When liquid samples are placed directly in an oven at any temperature above the boiling point, some of the desired parameters may leak out of the container and be lost. Therefore, samples can be evaporated before drying to constant weight by drying at a temperature below the boiling point using a steam bath, hot plate, or oven.

During desiccation, all samples should be given attention. The number of times the desiccator door is opened should be minimized to prevent the entry of moist air. If the dehumidification of the samples is stronger than the desiccant used in the desiccator, the samples may absorb water. In general, weigh samples immediately after removal from the desiccator to minimize the absorption of water from the atmosphere.

Drying at 103 to 105 °C may leave water of crystallization and some mechanically trapped water. Trapped water, organic matter and carbonates [released as carbon dioxide (CO2)] may slow down the drying process considerably. Drying at 180 ± 2 °C should remove mechanically trapped water, but may also lose organic matter and some salts.

Dry samples to constant weight. If possible, this requires several drying, cooling and weighing steps for each sample. Results for samples with high oil or grease content may be questionable, as it is difficult for such samples to dry and reach constant weight within a reasonable time. Any sample that does not reach constant weight should be qualified by indicating the number of drying cycles and the change in final weight.

If possible, dry samples to constant weight, taking into account weight changes due to exposure to air and/or sample decomposition. Ensure that samples are cooled to ambient temperature before weighing. Also, remove excess water from glass-fiber filters before placing them in the containers intended for weighing. Excess water will cause the filtered material to stick to the container during drying, which will cause the material to increase or decrease when removed for weighing, thereby distorting the results. This is especially important for low TSS measurements.

Use medium-grade distilled water (or better) for rinsing filters and filtered solids and for cleaning laboratory equipment. Special samples may require higher quality water (see Section 1080).

When an analysis deviates from the initial procedure for any reason, record the changes and submit them with the results so that the reasons for non-compliance with the method are documented.

Sample Collection and Storage

Use borosilicate glass, plastic, or fluoropolymer bottles. ] such as poly(ethylene fluoroethylene (PTFE), Teflon [ ® until suspended solids in the sample do not adhere to the vessel walls. Begin analysis as soon as possible, but in no case should the sample be held for more than 7 days. Settleable solids should be analyzed within 48 hours. Between collection and analysis, the sample should be kept cold (not frozen) at a temperature below 6 °C to minimize microbiological decomposition of the solids. Allow samples to reach room temperature before beginning the analysis.

Method Selection

Methods 2540B-F are suitable for the determination of solids in drinking, surface, and brackish waters, as well as domestic and industrial wastewaters. The analytical range for the 2450B-D is 2,5 to 200 mg/L for a 1000 mL sample, but may be increased by using a small sample volume for analysis. Method 2540G is suitable for the determination of solids in soils and sediments, as well as solid and semi-solids produced in water and wastewater streams.

Quality Control (QC)

The QC procedures considered an integral part of each method are available from reputable sources.

Analyze ≥ 5% of all samples in duplicate or at least one replicate with each batch of ≤20 samples. The laboratory may record the replicate measurements on a control chart for evaluation. Typically, the relative percent difference (RPD) of the replicates should not exceed 10% ,but RPDs may vary considerably depending on the sample matrix and concentration.

Analyze one MB per batch of 20 samples for each method (except for settle able solids (2540F)). The MB analysis includes all steps and procedures for preparing containers and filtration, except for adding the sample. If any of the blank measurements are at or above the reported level, take prompt corrective action (see Section 1020B.5). This action may include reanalyzing the sample batch. Consider one laboratory-level blank (LFB) per batch of 20 samples for all tests except for dissolved solids (2540F) and total solids, fixed solids, and volatile solids in solid and semisolid samples (2540G). Record the percent recoveries on a control chart for laboratory evaluation. Laboratories may prepare recognized standards or develop internal control methods for use.

Reference: Standard Methods for the Examination of Water and Wastewater

Translated by Maryam Soltani