leachate removal-gravity or pumping 1

[lauragray]

We are expanding our landfill, which currently has a gravity leachate removal system. We've experienced some problems with gravity removal and are investigating this. We may opt for pumping, either downslope submersible pumps or stationary with suction hose in the future cells. Does anyone have experience in this area that may help us decide which to choose? thanks, Laura

 

[RWF7437]

What kind of problems are you experiencing with the gravity system ?

As far as I know, gravity is free, never is turned off and operates 24 hours per day so is always the preferred method for conveying fluids. Pumping costs money, can fail mechanically or electrically. etc. etc.

Good luck

 

[lauragray]

We installed duck bill check valves in our 6 inch gravity lines from the landfill cells and we believe they are restricting flow to the collection point. We will be pulling these out soon to see their condition. There is a possibility that they've inverted further restricting the flow. If that wasn't enough of a problem, we've also seen our 'gravity' line air lock which blocked flow to our lift station. The 'gravity' line is almost flat and if we don't get enough head in the collection manhole (due to low flow from 6 inch gravity lines from teh cells) the leachate doesn't flow. Also, we believe that the air entered the line due to low flow into the manhole and the level dropping below the top of the 10" discharge line. We are looking at vents on the line, but they would have to be pretty high to prevent leachate overflow and probably can't use air release valves due to the gravity flow (no pressure) scenario. So, we are trying to fix the current problem and redesign the collection /discharge system for the next cells to keep the leachate flowing to the lift station.

 

[rberickson]

A believe your options are (1) increased gradient/slopes, typically >2-5%, depending on the permeability of your LCRS soil (sand/grave?). Alternately, in addition to the operations layer, the possibility of incorporating a high flow drainage geocomposite may increase airspace & cost savings of increasing the granular drain layer. I believe this will also also be more cost effective than a pump station.

Just my thoughts.

 

[bozwell]

Collection pipe slope is very important for gravity systems. We've seen similar problems with slopes in the 0.5% to 1% range. The regulatory requirement in the state of Pa. is now no less than 2% for the post settlement grade. Have not experienced problems with flow with this grade. Have experienced problems with pumps due to electric malfunction.

 

[ken103162]

I've seen the condition you've noted regarding air lock on the collection lines. One way to make sure that it is air locked is by examining the flow. If the flow surges, it is vapor locked.) If the pipes between the landfill and the collection manhole are completely enclosed (common) it may help to put in air relief into the pipe using petcock valves with 3/8 inch tubing to the ground surface. If there is a surcharge condition, it will show up in the tubing. If the problem is originating outside the landfill, I doubt that the problem is related to slope. Sanitary sewers are frequently placed at very low slopes. Any chance that you could have a collapsed pipe?

Regarding pumping systems, I have used downslope submersible pumps. pumps are removable. Works well with few problems. One big advantage, I've noticed that the most likely location for leaks in the liner system is at the penetrations. These systems eliminate penetrations through the liner system.

One wild thought, if you are noticing that leachate builds up in the landfill cell without good collection in the leachate collection system, you may be experiencing biological plugging in the sand/gravel leachate collection system above the primary system liner. If that is the source of the problem, :-(