The Case for PURAIN Jump Filters for Rainwater Collection

Ecovie is now the exclusive distributor for innovative Intewa water management products.  A centerpiece of our new products is the PURAIN line of rainwater collection pre-filters.  We are excited about the possibilities since we see the “Jump Filter”, as it is sometimes called, helps us meet our vision to spread adoption of rainwater collection as one way to help solve our water management challenges.

All experienced rainwater collection professionals know that good pre-filtration is a critical element to having a viable rainwater collection system.  Water entering the tank needs to be free from debris that can harm water quality and foul pumps and downstream equipment.  Some of the most important design considerations of a pre-filter are:

1.  Water Quality:  It nearly goes without saying that a good pre-filter must provide clean water to the tank.  The purpose of the filter is to keep debris like leaves, pine needles, asphalt particles (from roof), and other contaminants from getting in the tank.  All the pre-filters that I know of on the market do a good job of this, so there is not much point of differentiation on this point.  There is some point of debate on how fine the filtration should be.  The ASPE 63 standard specifies that pre-filtration should have screening to 1500 microns or less.  All self cleaning pre-filters I know of far exceed this standard.

2.   Water capture:  A good pre-filter maximizes the amount of water collected under all conditions for flow and debris load.  Filters are rated on capture efficiency which is the percentage of filtered water going to the tank versus the amount of unfiltered water going to the filter.  Simple basket filters have 100% capture ratings up to a certain feed flow rate.  “Self Cleaning” filters typically have less than 100% capture efficiency.  Capture efficiency ratings are typically based on testing with water that has no debris and with filter screens that are totally clean.

Since the objective of rainwater collection is to collect as much water as possible, capture efficiency is obviously a pretty important consideration!

3.  Maintenance:  Simple basket filters may have 100% capture when they are clean, but they require continual maintenance (depending upon debris load) to keep capture efficiency at 100%.  Self cleaning filters tend to have lower than 100% capture, but require less maintenance to stay clean.  Some self cleaning filters stay clean better than others.

If a pre-filter is blocked up with debris, it cannot collect rainwater.  Most property owners do not want to inspect or clean out their filter after each rain event to this make maintenance requirements another important design consideration!

 4.      Ease of Installation: Having easy connection of the filter to conveyance piping and minimal labor required for in-ground filters is a definite advantage.

Time is money in the rainwater system installation business.  Installed cost is a big deal to both the installer and the property owner who is looking for a good ROI on their rainwater system.

Ecovie has a lot of literal “in the trenches” experience designing and installing rainwater systems since 2009.  This has allowed us to see a lot of different situations in the field with various rainwater collection pre-filters.  In many situations, other pre-filters have worked well, but in others there have been some issues that had to be addressed.  In all cases, pre-filters have delivered good water quality to the tank, so that has never been an issue.  But, sometimes issue arise in water capture efficiency, screen fouling, and ease of installation.  Based on what we have seen over the years, we are really excited about the performance of PURAIN Jump Filters.

How does the Jump Filter Work??

Once you see how PURAIN filters work, you will see why they have big advantages in capture efficiency, maintenance requirements, and installation.

All PURAIN filter sizes work the same.  They have a rainwater inlet that brings water from the roof top to the filter.  Water cascades over the 800 micron filter screen.  Clean water goes through the screen to the tank and debris accumulates in the hydraulic jump zone.  Water capture efficiency is 100% until the flow rate is high enough that the water level in the jump zone reaches the overflow point.  Each PURAIN filter size is designed to reach hydraulic jump at a flow rate equivalent to around 1.0” per hour rainfall at the maximum rated roof square footage.  Here is a close up of the 800 micron filter screen:

It turns out that almost all rain falls at a rate of less than 1.0” per hour even in areas that have high intensity rain events.  Even though it may rain “cats and dogs” often in places like where I live in Florida, the high intensity rain does not last very long.  High intensity rainfall over 1.0” per hour accounts for only about 3% of the rain that falls in most areas.  This means that the PURAIN filter has an overall capture efficiency rating of around 98%.  Even though this will vary with geographic area, this high capture efficiency exceeds that of all other self cleaning filters.  Here is a typical capture efficiency curve for PURAIN (note that capture efficiency is 100% up to the hydraulic jump flow rate and almost the entire volume of rain fall at a lower rainfall rate).

I think that this is a very big deal because I am a bit of a zealot about capturing every last drop of rain.  The guys I have working with on installations can attest to that (right Jason?).

As I indicated before, all other self cleaning filters have some water loss even at very low flow rates.  Also, they require the filter screen to wet out before water is accepted to the tank which further reduces capture efficiency.  Some filters even require a minimum flow rate before water is accepted so that on a very light rain, water capture will be extremely low.  This is of particular concern where rainfall tends to be light.  Think of the light drizzle of the Pacific Northwest in the winter.  For you über rainwater geeks, check out this university study which compares capture efficiencies of the DN100 PURAIN filter to a couple other self cleaning rainwater pre-filters.  The results much higher capture efficiency for the jump filter when tested with clean water and water with debris like leaves.

Self Cleaning:

Now, let’s talk about the self cleaning action of the PURAIN filter using the hydraulic jump.

At about 0.5” per hour rainfall rate (or higher depending on roof size in relation to the filter), the hydraulic jump starts as water level reaches the reject port height, mimicking the action of a river rapids.  The turbulence flushes out any accumulated debris and the filter screen is washed vigorously.  As flushing occurs, water is still being accepted to the tank but efficiency drops.  We expect this to happen frequent enough to keep the filter clean but not for long enough to lose excessive water.

Here is a photo of hydraulic jump occurring in a DN300 (12” diameter feed pipe) PURAIN filter at a flow rate of 200 gpm:

Note the amount of cleaning turbulence even at this low hydraulic jump rate.  At this flow rate, capture efficiency is still 100%.  According to another university study, full flushing cleanout of this filter occurs at over 400 gpm flow.  Here’s a photo of that:

At this flow rate, the study showed all debris including small stones were flushed out.

Another point is that that filter screen is set a steep slant so that debris does not accumulate on the screen.  So, even if debris does accumulate, filter accept rate is not affected significantly.

We recommend inspecting PURAIN Jump Filters twice a year, but so far we have not seen any need for cleaning.

On the other hand, our experience with other self cleaning filters has shown that sometimes filter screens blind over after almost every rain event.  In almost all cases, screens require significant cleaning twice a year.  Some filters use cleaning showers to keep the filter screen clean which adds complexity (i.e. cost and maintenance) and still require periodic cleaning.  As I stated before, if the filters are not cleaned, capture efficiency is lost so there is a tradeoff between the PITA factor of cleaning the self cleaning filter all the time versus losing precious rainwater.  Simply put, PURAIN avoids these issues.

Sizing:

As with any filter all the talk above about capture efficiency and self cleaning is somewhat irrelevant if the wrong size filter is installed.  If too big a filter is installed, capture efficiency of a PURAIN filter will increase, but there may be a need to clean the filter screen depending on local rainfall patterns.  Other self cleaning filters will actually lose efficiency when oversized due to the constant reject of water.  An oversized PURAIN filter may lose some capture efficiency, but has been fairly forgiving with respect to handling excess flow.  Other self cleaning filters may not handle high flow with water being thrown out of the filter at high rainfall rates.  This may be a particular problem if the filter is installed in indoor utility areas where excess water sometime causes flooding and at minimum a big mess.

An easy rule of thumb for sizing is to use the manufacturer rated maximum roof sizes.  These tend to be somewhat conservative in terms of handling flow because the rating usually assume a 75% full inlet pipe.  However, extreme rain events may cause the flow to the filter to increase significantly above this level, which may lead to filter flooding.  PURAIN filters have a somewhat higher roof square footage rating at any given inlet/reject pipe size due to the filter height and screen mesh size.  Here are the roof size ratings for PURAIN filters:

PURAIN Model

Inlet/Reject Pipe Size (in.)

Accept Pipe Size (in.)

Roof Square Footage Rating

(sq. ft.)

DN100

4

4

3,750

DN150

8

6

10,760

DN200

8

8

21,500

DN300

12

8

43,000

DN400

16

12

64,500

 

Actual performance will depend upon local rainfall intensity, debris load, roof type, and other factors.  Regarding rainfall intensity, one way the filter may be sized could be to look at 5 minute rainfall intensity frequency and size the filter for your location so that hydraulic jump will occur 4-10 times a year.  This range would give you good capture efficiency as well as good self cleaning.

As with any rainwater system sizing, extreme events should not be used for sizing.  Filters should be sized to work well under the types of rainfall that happens most often and not 5 year, 25 year, or 100 year events.  Otherwise, filters will be sized much too large which leads to poor performance and excessive capital cost.  This sizing is in conflict with typical drainage pipe sizing which needs to handle these extremes.  To satisfy this conflict it is often necessary to design emergency overflow on the approach piping to the filter so that water flows to the filter normally when rainfall is normal and has an exit path during extreme storms.

Installation:

The PURAIN standard series DN 150 through DN400 have female connection points that make connection to SDR PVC drainage pipe simple, requiring no extra couplers as with other filters.  The DN 100 does require a 4” Fernco coupling on 2 of the connections.

The filters can be used inside or external to the tank.  We can take you through the various proven methods for doing this.

The HD series has an advantage over some other filters in that they can be direct buried, eliminating the need for a concrete vault.  Installation of an HD in ground filter is explained in our installation manual.

Summary:

As always, I welcome comments and questions over this post.  I believe that there is a significant amount of data and practical experience which indicates the PURAIN pre-filters are the best available for capture efficiency self cleaning, and ease of installation.  I encourage you to give strong consideration to PURAIN for any rainwater collection project.

A final reason I am delighted that Ecovie is now working with Intewa is that there is a pipeline of innovative product improvements lined up to make PURAIN jump filters (and the other products) even better.  In the coming months you can look forward to some product changes that will make the PURAIN filter more cost effective while maintaining the same product benefits

 

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