Case Study #3023 Wastewater Skid


A system of concrete settling tanks in a duplex arrangement to be able to manually clean one while the other is in operation and a third tank used for secondary settling.


In order to make this a more continuous process they rented a wastewater skid designed for 150gpm. The skid consisted of a duplex arrangement of #2 size bag vessels, each with one 100um bag with 4.5 sq. ft. of area. Flowing 150gpm per vessel then diverting the flow to the other vessel when they reached 25 psid of differential pressure. They needed to change a bag every 15 minutes or 96 times per day.

They decided to try it by opening both valves and flowing 75 gpm per vessel, both vessels flowing at the same time. They changed 60 bags per day.

The results were still a very labor consuming process that cost them 24-man hours per day just to change filters at 150 gpm.


Design a filter system for the new desired flow of 250 gpm with the desired filter change out of not more than once per week.

The Plan:

Analyze the Application:

What type of contaminate, PPM’s, particle size, particle distribution, pressure’s operating and differential, temperature, etc. What filter media, how does it load, how efficient, how effective, any different level/micron required etc.

Determine the Requirements:

Higher efficiency (100%) at 100 micron (existing bags were 70% @100 micron)

Increase flow from 150 to 250 gpm

Reasonable filter changes interval (Maybe once or twice per week)

Complete skid mounted package

Define the Specifications:

Filter Media: Polypropylene media rated 99% efficient @ 90 micron

Filter Vessel: 316 SS, 150 psi, ASME code filter vessels

Piping: Schedule 40, 316 SS 150# ANSI flanged piping

Valves: 316SS 150# flanged full port ball valves, manually operated

Skid: Painted carbon steel with forklift access from all sides

Select the Proper Filter Media:

Because of the narrow particle distribution in this application a filter design with high surface area and high efficiency was determin

ed to provide the best choice.

A Selection of the Strainrite MDXMF filter was chosen for a test because of the media’s high performance; 99% efficient @ 90 micron, and the 40 sq. ft. of filter area per element that fits into a single bag housing.

A test was run putting this MDXMF element into the rented bag housing.

They put one element flowing at 75 GPM in each housing. They lasted the whole day, versus changing the bag 30 times each and provided the superior level of filtration efficiency, 100% @ 100 micron that the customer desired.

Calculate the size:

With this complete test data, we designed a filter system to provide a flow rate of 250 gpm with the amount of filter area to allow them to change the filters once per week.


We provided the complete skid mounted filtration package they wanted with filter vessels, piping arrangement and valves. The filter elements meet their new efficiency requirements and are changed once per week during their weekly 4 hour shut down.

Customers Comments: “Just terrific, couldn’t be happier” Bill - D

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C.C. Day Company

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