A Self Cleaning Wet-well

The ‘Inside story’ on a radical design approach

Is the cleaning of a wet-well just a costly and troublesome fact of life? Or is there truly an effective, cost efficient alternative?

Hydro Innovations © 2009

Topics:

1. The Typical Wet-Well

2. Why does fat build-up happen

3. Some Cleaning methods

4. Some “automatic” alternatives

5. The solution

1. The Typical Wet-Well

The typical wet-well consists of a pair of submersible pumps sitting at the

bottom of a wet-well (or pit) that could be between 1.5m and 3.0m in

diameter. The pumps are fitted to guide rails, which guide the pumps onto

discharge elbows located at the base of the well.

The pumps cycle alternatively during inflow periods (generally heaviest around

meal times), and pump between the “on” and “off” levels in the well. These

levels are generally determined by the level of the gravity influent line (invert

level) and the level equivalent to the top of the pumps.

Preventative measures fail to capture these products

Grease, fats and oils enter the systems when grease traps or other

preventative measures fail to capture these products, or when operators of

food out-lets are not diligent, or have accidental spills. To a lesser extent, the

householder, not aware of the consequences of their actions, could be using

their sewerage system to dispose of waste oil products.

The pumping stations that will generally “see” the most inflow of fats and oils

will be those located near high concentrations of restaurants, food outlets and

shopping centres. It is at these locations that operators are generally most

concerned about the effects that a build-up of fat in their wet-wells will have

on odours produced and the consequent need to clean them.

2. Why does Fat Build-Up Happen?

Fat build-up happens in the wet well as a consequence of the specific gravity of

the fats and oils, and the design of the “typical” wet well and its “typical”

equipment.

The fats and oils, being lighter than water, will naturally float on the surface.

Now visualise the DESIGN of the typical wet well trying to cope with this. The

pumps start when the “on” level is reached in the well, and stop when they are

somewhere near the top of the submersible pump motors. At this level in the

pit, the well is still at its full diameter and the pump suction could be upwards

of 400-500mm below this level, giving the pump virtually no chance of

removing any of the oils or fats.

“Grease blanket”, then rides up and down

This “grease blanket”, then rides up and down with the water level, being built

upon with each and every inflow, with virtually no chance of any volume of it

being taken away by any of the standard pump cycles.

3. Some Cleaning Methods

Lowering the “off” level would seem to be a logical method of “getting” to the

fat with each pump cycle. This has 2 major drawbacks from being the perfect

solution.

Firstly, any lowering of the “off” level would expose the submersible

electric motor to possible cooling problems.

Secondly, and most importantly, any lowering of the level could cause

the submersible pumps to vortex and possibly become air-bound on

their next cycle. This naturally would mean a call-out for the operators

and a re-priming of the pumps. And in any case, lowering the off level

could still mean that the oils and fats still sit above the pump suction.

Problems require addressing odour issues and functionality

So, having written this approach off, all that is left for the “typical” system is to

clean the well after it has reached a point with odour or functionality issues

that require addressing of the problem.

The common approach here is to use a combination of chemicals to break

down the greases, high pressure cleaners and/or some sort of self priming

pump or “vac truck” to completely remove wet well contents.

Expensive, Labour Intensive and with O.H. & S. risks

Any of these things is not inexpensive. They also all require the removal of the

wet well “lids” and safety grills to allow access to the well. This is not only

another expense, but increases the O.H. & S. risks and brings into play the

confined spaces regulations.

It becomes a minimum of a 3 person operation. More likely 5 or 6 when you

consider the need to operate safety lifting equipment, operate the pressure

cleaner, and have the mandatory observer. It may also require an electrician to

disconnect pump cables to minimise the risk of cutting them with the high

pressure cleaning equipment.

Cleaning wet-wells this way, is an expensive exercise.

4. Some Automatic Alternatives - limitations

Automatic wet well cleaning systems

We have come across several “automatic wet well cleaning systems”, but they

are invariably complicated mechanically and/or electrically. They are generally

based on some kind of water jetting to wash down walls and/or churn up the

“fat blanket”, but then still rely on the station pumps or a surface mounted

pump to remove the fats.

Flushing valves

Another popular (but possibly not with operators) method is the use of various

flushing valves. These valves are supposed to stir up greases and fats so that

they mix with the water and can be pumped away. This sounds good in theory,

but oils and fats are still lighter than water and still need to have “off” levels

close enough to the pump suction to draw fat into the pump. This again runs

the risk of pumps vortexing and becoming air-bound on a subsequent pumping

cycle.

Reliance on equipment at the bottom of the wet-well

In addition to the limited success these types of systems have, they still rely on

a piece of mechanical equipment that is located at the bottom of the wet-well

to function properly. It is equipment that will require maintenance and repair

from time to time and just another thing in the pump station that can go

wrong.

5. The Gorman-Rupp Solution

The best solution is prevention (that works), rather than a cure (cleaning after

fat build up).

Gorman-Rupp self priming pumps, when used for the main pumps at the

station, can be “set up” to provide a “self-cleaning” function:

- without adding one extra piece of mechanical equipment to the

station

- without adding a single electrical component to the control

system

- and without having to enter the wet-well or even open the wet-

well lids.

The answer lies in taking advantage of the superior air handling capabilities of

the Gorman-Rupp self-priming centrifugal pump.

Provided suction conditions are right (and Gorman-Rupp Super-T series and

Ultra-V series pumps are designed to be able to prime to 7.5 metres), suction

lines can be set very close to the bottom of the well, and pumps run until they

lose prime (are basically ”sucking air”).

No chance for grease build-up

This means that virtually all of the contents of the wet well are removed

(including the oil and fat) with each and every pump cycle, not giving greases a

chance to build up at all.

To achieve this result, pumps can be retrofitted to existing wet wells with

conical shaped bases, or older style rectangular wet wells, as well as purpose-

built new ones.

In all these cases, because self-primers only need a small amount of space for

the suction lines to fit into, they can be positioned at the deepest and

narrowest part of the well to ensure that the surface area of un-pumped fluid

after the pumping cycle is much smaller than the maximum diameter of the

well. In some cases, the surface area remaining can be as little as one tenth the

surface area of the main chamber of the wet well, so at the very least, in this

instance, fats will take up to ten times as long to build up.

There are numerous other benefits to using self-priming pumps in wastewater

applications which include:

• Lower maintenance costs

• Longer service life

• Greatly improved O. H. & S. conditions

• Safer operation

• Virtual elimination of Confined Spaces issues

We trust this paper has been thought provoking and of value to you.

There are several other papers that Hydro Innovations can make available to

designers of pumping systems. We also have information discs and

presentations on these subjects.

Call us on 02 9647 2700 so you can have a real ‘person’ contact when you

have ANY questions on pumps and their applications. We’ll also arrange for

you to receive priority advice on any new technical developments, new

product releases, tips on cost savings and maintenance, etc. Our mission is to

be a valuable resource to help YOU do your job better.

Hydro Innovations General Manager & author of the white paper series, Garry Grant

Hydro Innovations © 2009