Volume of Lift Station?

[AHickman]

I am needing to design a lift station for an RV park we are working on. It will be lifting sewage into our main treatment trains that we've designed for the park. Also, keep in mind, this is a lift station for < 1500 GPD, and there are no design standards, as it's out in the county our the governing body does not have any requirements for lift station design. We can use normal septic tanks, premade basin, etc. We are trying to design this as cheaply and install-friendly as possible. I have no problems determining losses, pump sizing, floats, and spec'ing panels & gear, but I'm trying to battle the depth vs having more lift stations dilemma. I could put the whole park on one lift station, but the sewer line would come in 6' deep. Inversely, I could have the line coming in no more than 3.5' deep and install lift stations using cheaper components and it'll be a much easier install, but I'll need 4 lift station instead of one, which is more moving pumps, more moving parts, more maintenance. If anyone has any advice on that, I'm all ears! Also, I'm not quite sure what volume I should be shooting for. If I'm using the basins (I've been talking to keen pumps, they go up to a 48" basin), and I put this thing 6' deep, and my ADF is 1500 GPD, I'd need to go 22' deep total (6' to reach sewer line, 16' for storage) to achieve 1 ADF of storage, which obviously we can't do, as the costs would be outrageous. Are lift stations design on a ADF aspect, or is it most important to just have a duplex grinder pump setup and pray for no power outage (or get a backup battery setup?)?

Thanks for the help

 

[Heaviside1925]

I don't think you said it but maybe I missed it, but will the RVs be plumbed or is this for black or grey water dump stations?
One large one would likely be cheaper but the client losses any redundancy if it goes down for maintenance, so this maybe a cost/benefit for the client.
Another consideration would be demand times. I could see evening being the most, followed by morning, with sporadic use throughout the day. This could lend to your pump sizing.

 

[AHickman]

They will be plumbed directly to this. So, I guess my question is what should I be sizing the tank for and the pump for. The smallest grinder pump is going to dwarf any flow requirements, so I'm not too concerned with that. Mostly just a volume question.

 

[Heaviside1925]

I would consider the 4 smaller ones. If one goes out, the client just losses part of the RV park. Reduces distance and size and simplifies of the RV to lift station piping as well. Maybe a little move $ than 1 big one but I think the cost could be justified to the client. Maybe more maintenance but likely smaller, cheaper more available components, so over all upkeep may be less. As far as volume, 1-day min, 2-day max. The client would just need time to notify their RV customers and move them to another site in case a pump goes down. RVs should also have their own tanks, so if a station goes down, the RVs may be fine for a day or two without sewer access or the good'ol honey wagon can go around any empty the tanks. I wouldn't see a reason to be concerned with long holding ability.
My 2 cents from a mechanical

 

[bimr]

This is generally what you are looking for:

Lift station wet well design.
i. Ensure that the minimum wet well volume in gallons is 1/4 of the product of the minimum
pump cycle time, in minutes, and the total pump capacity, in gallons per minute;

iii. Ensure that wet well volume does not allow the sewage retention time to exceed 30
minutes unless the sewage is aerated, chemicals are added to prevent or eliminate
hydrogen sulfide formation, or adequate ventilation is provided. Notwithstanding these
measures, the applicant shall not allow the septic condition of the sewage to adversely
affect downstream collection systems or sewage treatment facility performance;

v. Ensure that a wet well designed to accommodate more than 5000 gallons per day has a
horizontal cross-sectional area of at least 20 square feet; and
vi. Ensure that lift stations are designed to prevent odor from emanating beyond the lift
station site;

 

[swazimatt]

It will depend on your contingency options. the council here works on 8hrs ADF as that is how long they expect a failure to be resolved ie how long it would take to get a technician there to assess the situation and worst case get a sucker truck there to start removing effluent.
weigh up your failure risks and provide mitigation solutions, eg duty and standby pumps will cover pump failure, connection for backup generator, what are the failure risks outside of the pumpstation? is the discharge field likely to become inundated and need to find somewhere else to get the waste to.

If you have someone onsite all the time and can get a sucker truck there quickly you can reduce the storage volume, if not the case increase the time. This can be done within the wetwell or auxilliary tanks or just a depression that can hold overflow before spilling into the waterways (nobody likes this option!).

We normally size the pump to meet PWWF for the system and then the size of the wetwell (volume between pump on and pump off) based on maximum of 15 starts/hour