The person designing this pipeline is going to earn his pay. Only liquids will be sent down the pipe, solids will be hauled down the mountain separately (like the state does). For general assumptions the liquids are effectively water. The term that best describes it is leachate or gray water. The elevation difference from the facility to the base station is about 3500 feet of vertical, convert that to pounds per square inch and that is about 1500 psi if the pipe is full at the base of the mountain. That pressure has to be dissipated somewhere before it goes into the leach field. If it is flowing continuously than there is some pressure loss due to friction in the pipe, the goal of a designer is to make the pipe big enough on the inside to keep the velocity of the pipe low enough not to erode the interior of the pipe. Too big of a pipe can be an issue as it really should stay full. If it was a perfectly straight sloped pipe top to bottom an oversized pipe may not be an issue, but this pipe is going to have high and low spots with variations in slope. Water is incompressible but gases are not and if the pipe is not full, pockets of gases (mostly air) will form and that air will compress randomly and despite constant flow going into the pipe, the flow will vary at the discharge due to the gas bubbles. Most large hydroelectric installs with long penstocks will have surge tanks prior to the turbine to settle this phenomenon out. The long penstock that runs along route 2 east of Montpelier has one of these surge tanks. The down side with a surge tank is the top of it has to be level with top of the inlet. Thus not practical with 3500 feet of elevation. If the flow in the pipe is intermittent, these flow dynamics get substantial and the concept of water hammer comes in where the pipe needs to be even thicker. Putting multiple Pelton wheels ( A form of a hydroelectric generator) along the pipe to reduce the pressure works when its flowing but useless if its not flowing. If the flow is metered in at the top to keep the line less than full, water hammer and surging will happen.
I used to have to deal with stock lines and water lines between the mills in Berline and Gorham with only 231 feet of elevation difference and 10,000 feet of pipe and it always amazed me how much the flow would vary with a constant flow going in at the top. a 18" diameter pipe would literally stop flowing for a couple of seconds and then start full flow. We spent a lot of our time and various consultants time trying to model the dynamics but just lived with the flow surges by discharging the lines in large surge tanks.
My guess is there will be a learning curve getting things to operate optimally. The landfill in Berlin at Mt Carberry had similar issues with leachate flow coming down off the mountain to the Berlin waste water treatment plant. I had seen some of the results on occasion where leachate surging lifted the eventually bolted down covers on manholes. Its far nastier stuff than what would be in the proposed line.