Pump Station before Septic Tank?? 1

[SPSUEngineer]

Here's the deal. I'm the civil site engineer on a school addition with an existing septic system we want to use.

The architect wishes to lower the floor elevation of the new addition and that means the sanitary invert will be lower than the existing septic tank I need to tie into. I can't lower the tank elevation because it is already as low as it can be for the nitrification fields.

Can I install a pump station before the septic tank and pump effluent into it? I could design the force main to keep the velocity low (+/-2.5 ft/sec). I don't have much expertise in this area. I've searched the code and a few textbooks but can't find an answer to this. It seems to me that a grinder pump would create a "slurry" and the tank wouldn't be able to do its job of separating solids from liquids. Is this correct or am I overthinking it?

I'm trying to do this without installing an additional tank if possible. I have a situation where I can't increase the capacity of the existing system without bringing it up to code and I don't have the land area available.

This is in the State of Georgia and under the jurisdiction of the EPD (flow rate over 10,000 GPD) if anyone knows of a code issue.

Thanks!!

 

[Spartan5]

It is not uncommon for single family residences to use an ejector pump to lift waste to a septic tank from a basement.

You are correct that the waste slurry could impact the function of the system. However, without knowing more detail regarding the design, age, and condition of the system you are expanding, and the existing and proposed flows, it is tough for me to comment on what you are proposing.

 

[SPSUEngineer]

Thanks Spartan. Let me expand a little bit. The existing septic consists of 2 nitrification fields and tanks. The first one was construction in 1966 and the 2nd in 1984. The overall site handles approximately 12,500 GPD of waste from a elementary school. Due to our constraints we have to try to tie into the existing septic system at it's present elevation. I cannot lower the septic tanks because they are already as low as they can be based on the elevation of the nitrification fields.

I need to know if it is common practice or allowed by code to put a lift station upstream from a septic system for a commercial/institutional development. Will the septic tank still work properly in a commercial environment if it receives effluent from a grinder pump? I know this is a fairly common practice for residential but I don't know about commercial.

 

[Ussuri]

The water authority where I'm from does this from time to time. Where possible displacement pumps are used, certainly not macerators. But they do also use centrifugal pumps. In order to avoid disturbing the septic tank and washing solids/scum through the tank the pumped line usually terminates in a break chamber so that the flow into the tank is by gravity. The break chamber is also used as a splitter if more than one tank are being fed.

 

[SPSUEngineer]

Ussuri,

Are you saying I should not use a grinder pump but use a STEP type system instead? My only concern about using a non-clog pump is the forcemain will have to be larger than the forcemain for a grinder pump.

I'm only going to be pushing 20 gpm @ 20 ft. TDH. I'm thinking my GPM won't be the issue; i'll have to size the pump to get a minimum 2.5 ft/sec velocity though a 1-1/2" or 2" forcemain. A non-clog pump would have to larger to to get 2.5 ft/sec through a 3" forcemain.

 

[Ussuri]

The reason they dont use macerators is because of the concern that the shredded material doesn't settle out and instead passes through the septic tank clogging up the downstream disposal system.

STEP is not an acroymn I have come across, so please excuse my ignorance. In my experience,displacement pumps tended to be used in the treatment plant itself. For pumping to the works, typically submersible pumps.

 

[Spartan5]

Given that your system is/will be substandard by the current code I would be leery of the "common practice" method of design.

In my opinion, you should be looking at what the design, current, and proposed loading rates of your nitrification fields are as well as the design, current, and proposed hydraulic retention times in your septic tanks. If the system is significantly oversized, particularly on the primary side (septic tank), then using a sewage ejector pump could be acceptable.

Grinding the waste increases the waste strength (BOD) and total suspended solids of the effluent headed to your nitrification fields unless you significantly increase the retention time in the primary treatment portion of the system. Using a solids pump has the same effect, but not nearly to the extent as using a grinder pump.

The common practice here is to avoid grinder pumps when pumping into septic tanks. If it can't be avoided (retrofit situation), we would probably oversize the primary treatment by a factor of 2-3 minimum and would utilize effluent filters which would have to be cleaned frequently.

A Septic Tank Effluent Pump (STEP) system sounds like it would be an ideal solution if you can add additional septic tanks and provided your nitrification fields are capable of treating the increased flow. In that case, all solids would be detained in the new tank(s) and the effluent would be pumped to the existing tanks.

 

[SPSUEngineer]

Thanks Spartan. We are taking all the items you mentioned into consideration. It is a very unique situation I'm dealing with.

I found an ideal STEP system by Zoeller. Does anyone have experience with Zoeller products? Are they on par with Fylgt(sp?) or Myers Pumps? Looks like they have been in business for a while.

Thanks for everyone's help.

 

[SPSUEngineer]

Here is the system I'm thinking about. It looks like it would be ideal. Its in dwg format.

 

[Spartan5]

I prefer Orenco's STEP equipment. Particularly their pump/filter vault:

http://www.orenco.com/eps/eps_pumpvault.cfm

Very easy to maintain and well designed.

Their risers and lids are rock solid too. They use a high head turbine style pump though which is probably overkill for your application. Long life out of the pumps though.

As far as the Zoeller goes, I've not seen a setup like that before with a filtered effluent pump. I'm interested in it. It's not on their website, or is it? Maintaining the filter looks like it might be a bit tedious as it looks like you have to pull the pump to get at it.

If you go the STEP route (or any route that involves risers to grade/plastic lids), I would recommend specifying a secondary security measure in the riser such as:

http://www.gag-simtech.com/index_files/Page602.htm

especially at a school. Lids are frequently left off, the screws holding them on strip out, and mowers (or wayward snow plows) always win the battle with them.

Most septic servicers would probably prefer an 18" riser over the inlet end of the tank.

 

[SPSUEngineer]

Thanks for all your help Spartan. I will check out the Orenco system.

Zoeller does have it listed on their website. It is in the products tab and they have a ton of information on it. The filter has a lifetime warranty.

I'll check into the security measure as well. That is definitely a good idea. Kids like to mess with things and I can see a mower grinding the lid up.

 

[bimr]

If your floor elevation is low, then yes you can add a lift pump. There should be no issue as long as your leach field as adequate capacity.

There is no reason to use a step system or a grinder pump. A standard non-clog lift station type pump is adequate.

You should design for a higher flow velocity of at least 3.5 ft/sec. Any lower velocity than that will eventually allow the solids to buildup as your pump starts/stops. A velocity of 2 ft/sec is adequate for self-cleaning as long as the flow does not stop. The higher velocity is necessary to resuspend the solids when the pump restarts.