Importance of a Weir...

The picture to the right demonstrates the use of a weir in the settlers used by Black Dog Analytical.  The settler weir allows processed wastewater to exit the settler and prevents short-circuiting of the system. As wastewater rises from the bottom, it goes over the weir and returns back to the flow channel through the discharge hose (shown above as the green hose). This allows a continuous and uninterrupted flow of the process wastewater through our system.

Now let's look at a settler that is designed to work...

In order to settle a 50-micron grit particle, a settler overflow rate of 3-gpm/ft^2 of surface area must be achieved.  For our 55-gallon settler, this is ≈ 10 gpm          (πr2=3.14 x 3 = 9.42 gpm).
First, notice that the settler is equipped with an inlet valve (blue handle) that allows flows to be maintained throughout the sampling period.  This is usually between 7.5-8.5gpm, which is slightly lower than 10gpm, but serves as a safety in case extremely light grit is present.
Second, the sample is introduced into the side of the settler and diverted at a right angle to kill the velocity and allow grit to settle to the bottom.

Lastly, there is an air vent (white PVC tube sticking up) that allows fresh air in to the discharge hose.  This eliminates the formation of a vacuum in the line and allows more consistent flows through the settler.  Additionally, the settler is equipped with a discharge hose that directs overflow back into the channel, as well as a drain valve (blue handle lower left of 55 gallon drum) to decant the supernatant when retrieving the sample.

Split sample methodology

The Black Dog Analytical sampling method utilizes a split sample methodology, which allows the most uniform flow characteristics and minimal disturbance in the natural flow of the plant.  First, high-flow trash pumps (pictured top left) pull water from the main flow channel and into our sampling and settling system.  Water enters from the left and exits on the backside where a splitter is placed in-line to split a fraction for sampling. 

Pictured on the bottom left, wastewater enters on the right-hand side of the trash pump, flows through the pump and exits through the green hose on the left side of the splitter. This main green hose returns the wastewater back to the flow channel uninterrupted.

A small sample is split off from the main flow path and directed to the right (small blue hose) and enters the settlers described above.  This split-sample method allows us to have a very wide range of control measures in place to adjust for poor flow, poor grit chamber performance, weather-related high flow events, or any other scenario that requires adjusting to the system flow.

Sample flows (blue hose directed to the settler) are monitored real-time by Black Dog Analytical and adjustments made to maintain the constant rate of flow described above.  

Problem #2 - There is no way to regulate flow.  A siphon-style sampler supplied with a one-inch suction hose and dropped off a 20-foot structure will produce about 25 gpm of flow.  In order to settle 50-micron grit, a feed rate of 10-gpm or less is needed.  

These two problems create the illusion of a course distribution that often lacks material less than 300-microns.  Ultimately, this will lead to the design of a grit removal system that will not work.

Run Away!!!!!!

Grit Settlers​

Moving forward.

Future progress in the area of grit settlers is pointing towards a conical bottom tank that allows the concentration of the settled grit particles in the bottom of the tank.  This will allow a more efficient decanting of excess water and greatly increase time efficiency when obtaining the final sample.  As can be seen in the image on the right, wastewater enters the conical drum towards the middle of the drum.  Notice it still has flow control via the small hand valve (red handle).  It also still maintains the 90-degree turn as it enters the settler in order to kill velocity.  As grit particles settle, they are concentrated in the funnel portion of the drum and the remaining water continues to flow up, over the weir (described earlier) and out the discharge hose and back into the main flow channel.

First, let's look at an example of a flawed settler design.  There is nothing wrong with using a large garbage receptacle for a settler, however, the same rules for design apply for any large container.

Problem #1 - The sample entry point is incorrect.  Flow entering this settler creates a vertical disturbance that does not allow for fine or light grit to settle.  This grit is washed over the side and is not considered in the final distribution.  A properly designed settler introduces the sample into the side of the unit and approximately halfway down. A ninety-degree elbow directly inside the settler diverts flow to the side of the settler, ultimately killing the velocity and allowing fine grit to settle.