Methods

Black Dog Analytical is capable of employing any sampling method our clients require. There are no standardized methods of sampling grit, and our methods constantly evolve as improvements are discovered. Manufacturers of grit removal equipment often have a pre-determined sampling protocol which we follow if required. If no sampling protocol is in place, our methods are determined by sampling site access, which is generally determined prior to the site visit. Ultimately, our clients decide which method we use.

Obtaining a representative sample from the waste stream

This involves selecting an appropriate method of removing a homogenous cross-section of wastewater from the waste stream. Several sampler designs exist, and each has its place depending upon site conditions. Black Dog Analytical samples continuously for six to eight hours and brackets the morning peak flow when the most grit is transported to the facility.

are placed in the waste stream, preferably in areas lacking deposition. The samplers are periodically moved from side-to-side to capture a profile of the entire waste stream.

A suction hose is attached to the sampler, typically a two-inch hose equipped with cam-loc fittings. The hose is attached to a gas-operated trash pump that removes a large volume of sample from the waste stream.

Separating the grit from the sample

Because a large volume of sample is being removed from the waste stream, a small portion is split from the trash pump discharge hose. This portion is fed to a .

A 55-gallon drum equipped with the appropriate fittings and fed less than ten gallons per minute will settle grit as small as 50-microns. See the section labeled for a discussion on settler design and learn the difference between a good and bad grit settler.

The sample is supplied to the settler continuously and the flows are adjusted to maintain a consistent flow rate between 7.5 and 8.5 GPM.

Once the sampling period ends, the settler contents are allowed to stand for twenty minutes. The supernatant is discarded and the remaining grit/sludge is decanted into a bucket.

The single bucket is parceled into several buckets and rinsed to remove any excess organics. The cleaned sample is now ready to analyze.

Wet Sieving

The collected and cleaned sample is rinsed through a series of .

Wet sieving prior to ashing avoids “fracture” that occurs when exposing some materials to high temperatures. Silica sand will not fracture, however grit is comprised of a variety of materials and varies regionally. Surprisingly, some people actually think grit is just sand.

Wet sieving also allows any material attached to the grit particles to remain. These materials, such as fats, greases, and other Surface Active Agents (SAA) may affect the settling velocity of grit.

Physical Size Distribution

Once settling tests are complete, the fractioned samples are dried and ashed, leaving only the inorganic portion of the sample
These values are plotted and comprise the Physical Size Distribution

Sand Equivalent Size

This test is performed on-site immediately after sieving. Sand Equivalent Size (SES) generates a value that includes all the variables that may affect the grit particles including specific gravity, composition, attached organics, and shape.
Click here for a further discussion of SES