Refers to a solid that is suspended in a liquid. Most treatment facilities for both municipal and industrial wastewaters must meet effluent standards for total suspended solids (TSS). A typical TSS limit for a secondary wastewater treatment plant is 20 mg/L. However, some industries may have permits that allow them to discharge much more, for example, 500 mg/L. The test for TSS is commonly performed by filtering a known amount of water through a pre-weighed glass-fiber or 0.45 micron (μ) porosity filter. The filter with the solids is then dried in an oven (217°–221°F [103°–105°C]) and weighed. The amount of dried solid matter on the filter per amount of water originally filtered is expressed in terms of mg/L. Solids which pass through the filter in the filtrate are referred to as dissolved solids. Settleable solids (i.e., those that settle in a standard test within 30 minutes) are a type of suspended solids, but not all suspended solids are settleable solids.