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inflow to sanitary sewer


Not only did FAU host the ASCE southeast regional competition in March, but I have had a big deadline – my next book is due the end of this month to the publisher.  That has taken a lot of time, and I have had several issues divert my attention at critical junctures.  Fortunately the book is nearly complete.  I should meet the deadline.  This book should be topical.  It is about infrastructure management.  JRoss is the published and with a little luck it will be out in time for the holiday sin 2017.  Very good stuff.  The first part of the book focuses on the benefits=of infrastructure tot eh economy.  They are intrinsically lined although there is an interesting research project needed to study how much infrastructure creates economic growth and how much growth requires more investments.  Is there a point of diminishing returns.  Paul Krugman may want to weigh in as I did quote him a couple times.  Then the local systems are discussed – what can happen, maintenance needs etc.  Water, sewer, stormwater roads are featured. Lots of pictures and some means to autopsy the issues.  The rest of the book looks are how to develop a system to manage the assets, value them, evaluate condition and fund improvements.  Work order are really important for causal factors.  What fails, and how often.  I think we can predict the problems.  My initial analysis, included in the book says we can with limited data.  Going back to those Bayesian roots.  Another project I would like to look at.  Finding the next Flint is a third project.  So many ideas, so little time (and no money to get support).  The solutions will involve leadership, so I did insert some future risks and past “what could possible go wrong” issues.  Sorry Flint, you made the cut, but so did Alamosa, Walkerton, midwestern power companies, and my friends in St. Pete.  But instructively I hope.  The book is aimed at professionals, but a student teaching guide will be developed this summer for use in the classroom.  Should be fun.  700+ references. And I could add so much more, but I think it will diminish the usefulness.  No doubt it will make the best seller list – looking forward to my name on the NY Times best seller list.  LOL.   Or at least sell enough copies to make JRoss interested in another book.  But seriously it should prove interesting.

 

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The other thing we learned was that we need to be far more careful about what goes in the sewer system.  Paper towels, baby wipes and hand towels do not deteriorate in the sewers.  No matter what manufacturers claim, you find them everywhere and they look just like they did when flushed.  They clog lift station pumps and pipelines.  Do not put these down the toilet for any reason?  Likewise there are no feminine hygiene products that should be flushed, ever!  Again regardless what the manufacturers claim, you can find there ubiquitously in the sewer system and they look, well just like they did when flushed.  No biodegradation.  I have included some figures.  They show up in pump clogging and at plants as well.  They are not biodegradable.  Again do not put these down the toilet!  Put all these products in the trashcan in the bathroom.

Worse, do not put grease down the drain.  One photo is a greaseball in a manhole.  It fills the whole manhole up!  Of course the feminine hygiene products, towels, wipes, etc. plus grease make almost impenetrable obstacles that block the sewer system.  So we need to remove the inflow and we need to keep grease and the reset of these products out to reduce the costs of operating the wastewater utility.  We all contribute, and we all can help.  We want systems to operate properly and dependably, so let’s do our part.

photo 3 photo 1 GREASE


Wastewater utilities and water utilities are intrinsically linked.  Wastewater utilities often discharge to water bodies that are water supplies for downstream water plants.  In other cases, wastewater plants provide additional supply options to reduce water demands in the form of reclaimed water.  However as a wastewater utility, costs are often associated with power- pumping and aeration, which can be 30% or more of the utility’s costs in the worst cases.  However, substantial savings in operations can be achieved by reducing the amount of wastewater that must be pumped and treated and in some cases that reduction also is associated with water quality benefits for the reuse of reclaimed water.  Utilities have long dealt with the infiltration and inflow (I and I) issues in their system by televising their pipes and identifying leak points, but this primarily addresses only the infiltration part of I and I.  Inflow and infiltration are not the same thing – they are very different and must be addressed differently.  Inflow causes hydraulic issues during rain events – like sanitary sewer overflows and basement flooding.  Both subject the utility liability from lawsuits and/or regulatory fines.  Inflow is the risk issue that must be addressed to protect the utility.  A cost effective solution to inflow involves low tech, low cost methods can identify the problems that can corrected easily.  Removing the inflow portion from I and I, often leads to a more focused plan for infiltration correction.  What are those tools?  Smoke testing, cleanout repairs, sealing manholes and manhole dishes.  But each of these needs to be carefully selected.  Because these solutions, pipe that leak can be seen through another low tech solution – a midnight monitoring event.  Recent efforts here in south Florida indicate that only 15-20% of the pipes in a sewer system need to be televised and within those, about half the leaky pipes are actually not leaking – they are broke laterals.  Laterals are one of the most ignored parts of the sewer system – often they are small pipes and much of the piping is on private property so the utility does not address those pipes.  And in many utilities these are the pipes in the worst condition.

Other things that our efforts have shown are that new pipe can leak, just like old pipe, clay is not the only pipe that leaks and that the inflow solutions can be very helpful.  Figures 1-4  show how the solutions affected three lift stations and one community.  The graphs show rainfall vs flow.  Before these efforts, the flows increased with rainfall events.  After, they did not.  Hence this utility was able to resolve its risk for overflows at a cost of under $500/manhole.  That is relatively inexpensive.

LS 52 db LS 54 LS 53


Storms highlight the need to reduce infiltration and inflow into the collection system so as not to overwhelm the piping system causing plant damage or sewage overflows into streets, so much of the focus has been on dealing with removal of infiltration and inflow through televising the sewer system and sealing or lining sections where leaks are noted.  However, many miles of videotape show virtually nothing, so significant money is spent to find “nothing.”  Part of this is because “infiltration” and “inflow” are not the same, and storm events do not highlight infiltration nearly as much inflow.

The manholes and clean-outs are required for access and removal of material that may build up in the piping system and for changes in direction of the pipe.  Manholes are traditionally pre-cast concrete or brick, with brick being the method of choice until the 1960s.  Brick manholes suffer from the same problems as vitrified clay sewer lines – the grout is not waterproof so the grout can leak significant amounts of groundwater.  The manhole cover may not seal perfectly, becoming another source of infiltration.  Pre-cast concrete manholes resolve part this problem, but concrete is not impervious either.  While elastomeric or bituminous seals are placed between successive manhole rings, the concrete is still exposed.  Many utilities will require the exterior of the manholes to have a coal-tar or epoxy covering the exterior which helps to keep water out.

Inflow results form a direct connection between the sewer system and the surface.  The removal or accidental breaking of a cleanout, unsealed manhole covers, laterals on private property, connected gutters or storm ponds, damaged chimneys from paving roads, or cracking of the pipe may be a significant source of inflow to the system.  All are potential sources of inflow which can be identified easily during storm events.  The peaking that correlates with the rainfall is inflow, not infiltration since infiltration is part of the base flow that creeps upward with time.  When operators see peaks, this is not indicative of infiltration which is groundwater.  Think inflow.   Inflow causes peaks in run time on lift station pumps, and create potential overflows at the plant.  The good news is that simple, low tech methods can be used to detect inflow, which should be the precursor to any infiltration investigation.

The following outlines a basic program for inflow detection and correction for any utility system.  The order is important, and pursuing all steps will resolve the majority of issues.  The first step is inspection of all sanitary sewer manholes for damage, leakage or other problems, which while seeming obvious, usually surprises.  The manhole inspection should include documentation of condition, GPS location, and some form of numbering if not currently available.  Most manholes have limited condition issues, but where the bench or walls are in poor conditions, that should be repaired with an impregnating resin.

Next is repair/sealing of chimneys in all manholes to reduce inflow from the street during flooding events.  The chimney includes the ring, cement extensions, lift rings, brick or cement used to raise the manhole ring.  Manhole covers are often disturbed during paving or as a result of traffic.  The crack between the ring and cover can leak a lot of water.  The intent of the chimney seal is to prevent inflow from the area beneath the rim of the manhole, but above the cone.

The next step is to put dishes into the manholes.  One might think that only manholes in low lying areas get water into them, but surprisingly every manhole dish that is properly installed has water in it.  Hence assume that all manholes leak water between the rim and cover.  Most collection system workers are familiar with dishes at the bottom of the manhole where they are of limited use.  This is because the dish deforms when filled with water or is knocked in when the cover is flipped.  The solution is a deeper dish with reinforcing ribs.  No ribs, don’t use it.  A gasket is required.

Once the manholes are sealed, smoke testing can identify obvious surface connections.  The normal notifications, inspection and documentation will identify broken or missing cleanout caps, surface breaks on public and private property, connection of gutters to the sewer system, and stormwater connections.  All should be documented via photograph, by associated address and public or private location. The public openings at cleanouts can be corrected immediately.  However, if the cleanout is broken, it may indicate mower or vehicle damage, that can occur again.  If missing, the resident may be using the cleanout to drain the yard.  In either case the collection system needs to be protected.  USSI (http://www.elastaseal.com/about_us.html), located in Venice, FL developed a solution, called the LDL plug to correct those commonly broken or commonly opened cleanouts to reduce inflow.

Notices should then be sent to property owners with documentation of the inflow connections to their property.  This is sometimes the most difficult part of the program due to political will, but it is necessary.  This finishes the inflow correction portion of the project, but one more step will help focus efforts for the second “i”.

The final step is a low flow investigation, which is intended to focus on the infiltration piece of the problem.  Such an event will take several days and must be planned to determine priority manhole to start with and sequencing.

Based on a projected plan and route:

  • Open the manholes
  • Inspecting them for flow
  • Determining if flow is significant.  If investigation of basin will end and new basin will be started.  If flow exists, open consecutive manholes upstream to determine where flow is derived from.  Generally a 2 inch wide bead of water is a limit of “significant” infiltration.

Documentation of all problems and corrections in a report to utility that identifies problem, location and recommended repair.  Identification of sewer system leaks, including those on private property (via location of smoke on private property).

The example in Dania Beach, FL was that the last step indicated that only 15% of the sewer system needed to be televised.  This saved the City almost $1.2 million.  Their total costs is under $1.4 million for all parts of the project, spread over several years and contracts.  Overall the hope is that the inflow and infiltration programs together will save $400,000/yr, a five year payback.  But the key is to insure you get the inflow as well as the infiltration… Otherwise storms will continue to overwhelm plants, creating public health concerns and ruining your reuse program.

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