Advanced pumping technology for liquid/solids separation and clarifier processes in treatment plants

 

At a waste treatment plant or a solids precipitation facility, clarifiers remove solids efficiently from liquids, creating a sludge or solids layer at the bottom of the clarifier. The sludge in a waste treatment plant can be approximately 5 percent solids by volume based on a million gallons per day and 200 parts per million solids in the waste stream. This layer of accumulated solids forms slowly at rates of less than 2 gallons per minute based on a continuous flow of approximately 1 million gallons per day.

The clarifier is one of the primary solids removal systems within the plant. Expensive chemicals are sometimes added to the waste stream to enhance the sludge separation by bonding with the solids, which causes increased settling and a greater sludge density. Yet the predicted sludge density is not always met when physical measurements are made. 

 

Predicted and Actual Sludge Density

There is a difference between predicted to actual sludge/solids density because the sludge blanket at the bottom of a clarifier is thicker and more viscous than the liquid above it. As a result, a sludge depression cone can develop in the sludge blanket allowing the thinner liquid from above to flow from the clarifier instead of the more viscous sludge (see Figure 1). This occurs when too much sludge is drawn from the clarifier too quickly causing a suction cone around the discharge outlet of the clarifier. The less viscous material will be drawn in place of the denser layer.

 

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Figure 1. Clarifier sludge flow

 

This in turn decreases the total solids load and increases the amount of liquid sent to the digester. This defeats the purpose of the clarifier, increasing the total treatment cost. This can be demonstrated by measuring the sludge solids load at different times during a single pump operating cycle. At the beginning of the pump cycle, its load can be greater than 5 percent, and at the end, it can be half that figure. 

This variation of sludge withdrawal is caused by cone development from inefficient pumping. The thinner liquid will follow the path of least resistance. The cone will remain through the complete pumping cycle and will only begin to fill in when the pump is stopped.

One remedy to increase the clarifier efficiency is to use a smaller pump or operate a large pump at a slower speed to limit the outflow and stop the cone from forming. The sludge to be removed should be thick and loaded with solids, and this requires a larger pump to move the higher viscosities and larger solids. The larger pump operating slowly will tend to clog. A small pump cannot handle the solids. It shears the sludge and has difficultly with the initial viscosity. As either pump continues, they both draw in more of the thin upper layer which reduces the total solids and increases liquid volumes, decreasing the total process efficiency.

Exacerbating the problem is the large piping that is required. The solids and subsequent clogging issues require a discharge connection from the clarifier and associated piping. This, in turn, causes low velocity in the piping with excessive sludge settling within the pipes and maintenance issues. Again, costs increase. 

 

Solution to Inefficient Solids Withdrawal

One normal solution for inefficient solids withdrawal is to run a large pump on a time cycle that is short enough to avoid cone development. Another is to try to match the speed of the oversized or undersized pump to the sludge formation in the clarifier. Unfortunately, the sludge and its formation is not the same from week to week or even from day to day. Sludge formation at the bottom of a clarifier is inconsistent. Therefore, the flow requirements vary. Neither of these solutions works well and requires constant attention and adjustment.

The solution to the inefficiency of solids withdrawal is to move the sludge at a rate equal to its accumulation at the bottom of the clarifier without creating a depression cone. This should be accomplished no matter what the sludge formation is on any given day and still maintain significant turbulent flow in the piping to eliminate clogging issues. 

What is needed is a pump with tailored flow characteristics that match the individual settling characteristics of the sludge and the hydraulic characteristic of the process. This will maintain the sludge/solids at their maximum density, which in turn, improves the retention times, reducing unnecessary recirculation of liquid and improving the operation of the secondary or anaerobic digester. This significantly reduces the plant operating costs for energy, maintenance and ultimate solids removal costs. 

The operating characteristics of single-diaphragm air-operated pumps address these issues (see Figures 2 and 3). The pump cycle rate or speed can be matched to the rate of accumulation in the clarifier by means of a 4-20 ma signal from a sludge blanket level detector. This will speed up or slow down the pump directly to prevent the depression cone formation and maintain a constant sludge level. Because of the variable reciprocating motion of the pump, a continuous acceleration change of the flow occurs in the pipe. This creates turbulence, even in 12-inch pipe, that will keep most solids in suspension. 

 

Figure 2. The discharge stroke of the single-diaphragm air-operated pump. Figure 3. The suction stroke of the single-diaphragm air-operated pump.

 

By adjusting the air pressure that controls the upward and downward motion of the diaphragm, both the discharge stroke and the total strokes per minute can be varied. This changes the reciprocating motion of the pump and liquid independent of the total gallons per minute required by the system. The pump becomes tailored to the requirement of the individual application every day, eliminating cone formation and the subsequent low piping velocities while keeping the solids in suspension. This, in turn, reduces energy use and lowers maintenance time and costs, creating greater reliability, better sludge movement and lower total operating costs. 

 

Benefits of Single-Diaphragm Air-Operated Pumps

How much can be saved by using this type pump? If the solids by volume are increased by 50 percent, the total volume of liquid is cut by 50 percent. By example, the underflow of a clarifier is 5 gallons per minute at a 2 percent solids load. If the solids volume is increased to even 4 percent, the underflow becomes 2.5 gallons per minute, carrying the same volume of actual solids. 

There is half the volume of liquid to treat in the secondary digester reducing the liquid carryover by half. In an anaerobic digester process, half the amount of heat is required and a much greater retention time, better gas production and higher percent solids load is achieved. 

As stated earlier, the single-diaphragm air pump gives excellent control of the process underflow and consistency of the sludge. This allows for continuous sludge transfer, which greatly enhances control and creates few system upsets. Because the solid load is more consistent, chemical addition can be more constant, reducing usage and costs. 

Since the temperature and chemistry of the system are more stable, the bacteria cultures are healthier with higher counts that create better gas product and higher efficiency. In a waste activated system, the supernatant from the digester that is returned back to aeration is reduced greatly, therefore, reducing blower costs. The digested sludge or precipitate is more uniform and better for land application as a soil enhancer or conditioner.

These pumps are designed to handle the difficult liquids, which are both abrasive and corrosive (see Figure 4). They help solids remain in suspension within the piping and gently transfer even the most shear sensitive products without damage. Whether they are used on a clarifier or on any solidification process, the single-diaphragm pump will save money in maintenance, reliability and process enhancement. This design will make the plant more efficient and reliable, lowering life-cycle costs.

 

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Figure 4. A single-diaphragm air-operated pump in a clarifier underflow application