Contents

ADVANCES IN STORMWATER & EROSION CONTROL PRACTICES

INFLOW AND INFILTRATION PROBLEMS

NEW DESIGN FOR ANAEROBIC DIGESTER COVERS

GLOBAL POSITION SYSTEM REVISITED

BRIDGE PAINTING - WHY, WHEN, HOW, HOW MUCH ($)?

CLASS HP CONCRETE

by Glenn D. Cooley, PE

Stormwater management and erosion control are more and more important as developments become more dense and protection of streams more critical. Years ago, no one thought much about runoff from construction projects; so what if some topsoil washed away, you could always buy more. However, today that situation is both economically and environmentally expensive. Currently most municipalities with site plan review regulations utilize the "New York, Guidelines for Urban Erosion & Sediment Control" manual to set standards for erosion control and require stormwater systems to be designed to prevent an increased rate of runoff after development. Copies of the manual are available from the Empire State Chapter of the Soil and Water Conservation Society.

Stormwater management normally involves routing all runoff from a site (retail plaza, mall, subdivision, etc) to a limited number of outfalls (usually one) to streams or water courses. At this one outfall the maximum flow rate is restricted to the same rate that occurred prior to the development. Where land area is not a problem the excess water is stored in open detention ponds. These ponds can also reduce the pollutant load by settling out silt and grit. In some instances an open pond is not feasible due to space limits or aesthetics. In these cases water can be stored in the stormwater piping system by oversizing the pipes. This can be enhanced by using perforated piping, if the underlying soils are porous, to permit infiltration of all or some of the runoff.

There are several companies that make high density polyethylene "chambers" that can provide a larger underground infiltration area. Two of these companies are Advanced Drainage Systems, Inc and Cultec, Inc.

In some extremely cramped areas (or for other reasons) it may be impractical to control the rate of stormwater runoff. However the stormwater can still be treated to reduce its pollutant load by use of underground oil, grease and solids separator tanks. Several manufacturers produce precast concrete tanks that have built-in baffles and pipes to provide the separation. Most use a whirlpool action to separate grit from the flow stream prior to discharge. Each design has unique baffles to keep floating debris and oils and grease contained in the unit. Since the separators store the removal materials they must be cleaned out regularly or they become ineffective. Companies making these tanks include Vortechnics, Inc., H.I.L. Technology, Inc. and Environment XXI.

Post construction erosion control includes revegetating the site and open ditches and swales. Erosion in ditches due to water velocity and steep ditch slopes often cannot be stopped by grass alone. In the past, ditches were lined with concrete, shotcrete or grouted rip rap. You have all seen cases where excess water flow eroded around and under such "hard armor" type ditch protection. Where velocities allow, "soft armor" erosion protection can be used and is more economical than hardened ditch linings. The most conventional soft armor lining is light or medium stone fill. These are crushed rock of varying sizes placed over a geofabric liner. Another form of soft armor is the turf reinforcement mat. This is a coarse blanket of nylon or PVC fibers which is fastened to the soil in a ditch so that the ditch bottom and sides are covered. Grass seed is then sown over the area and allowed to grow. The non-biodegradable mats are a permanent reinforcement for the sod. Soft armor has the added advantage that as the ditches may settle the armor material moves with the soil keeping the protection in place. Two manufacturers of such material are Akso Nobel Geosynthetics Company and American Excelsior Company. Soft armor can also be used in conjunction with hard armor to protect the sides of ditches above the hard protection limits.

INFLOW AND INFILTRATION PROBLEMS

by: Allan R. Vanderpoel, P.E.

The most common problem facing sewage treatment plants in this part of the world is too much flow. The source of the excess flow is very often a problem with inflow and infiltration of storm water, rather than a problem with an overload of sanitary sewage.

The consequence of not removing the inflow or infiltration will likely be an order from PADEP or NYSDEC to enlarge the treatment capacity of the sewage treatment plant. This is an expensive proposition, as anyone who has worked on a sewage expansion project can attest to. Bills to the systems users can quickly go to $35 per month, or higher to fund this work.

The elimination of inflow and infiltration is not an easy problem, but current technology will help. A systematic plan to correct the problem is the most essential task.

The first step involves a flow monitoring study to determine which areas of the system have excessive storm flows. By simply observing each manhole in dry and wet weather, a good idea can be obtained on where the worst problems exist. A thorough record keeping system to indicate the depth of flow in each manhole and any observed leakage into the manholes is essential to this step.

An analysis of the observed flows will lead to the next steps. The installation of flow meters at some key points will allow accurate measurements over the years to monitor the success of the work. Flow meters have undergone a great technology leap in the past few years, with computer downloading of the recorded data one option. The cost and application will determine exactly what type of flow meter to install.

Smoke and dye testing, and televising lines is the next step in the analysis. Smoke testing is the first of these recommended, and very simply, where smoke comes out of the ground, storm water will go back in. The smoke testing needs to be done in dry ground conditions, with an adequate number of people to record each point of smoke in a few minutes time period. Dye testing is a methodical but definite method to confirm entry points. This is especially valuable to double check catch basins and downspouts that have in ground discharges. Smoke will not travel through a water filled trap, but the dye testing easily confirms this. Televising lines is the most expensive of the three tests, but tells a visual story of otherwise inaccessible lines. Pipes that have high wet weather flows where smoke and dye testing cannot confirm the problem are good examples of where to televise.

Concurrent to the observation and testing will be repair of the known problems. Inflow is the easier of these, as it involves plugging specific leaks. Once an observation of a known entry point is made, either patching or removing an illegal source of storm water is a positive step. This can be started as soon as inflow problems are detected, and by the end of the study, a list of known problems can be worked on. This may be harder than it sounds if several storm catch basins need to be rebuilt to divert flows from the sanitary system.

Infiltration will probably never be completely eliminated without a total reconstruction of the system. However, grouting, slip lining and reconstruction of the worst lines will help minimize the problem. And finally, monitoring through the use of the flow meters is essential to keep the flow problems from gradually reappearing. Only by a constant vigilance of the flow data can the individual sewer lines be kept free from new sources of inflow and infiltration.

NEW DESIGN FOR ANAEROBIC DIGESTER COVERS

by Jeffrey C. Bahret, PE

Across our country, thousands of anaerobic sludge digesters have been constructed for municipal wastewater treatment plants over the past 50 years. The vast majority of these digesters were built prior to 1965 and employ the floating rigid frame type of cover. The basic floating cover is a rigid steel structure designed to float (like a ship) on the liquid contents of the digester tank. Essentially, the floating cover consists of an integrally welded steel ceiling plate over the entire liquid surface of the tank. This ceiling plate slopes radially upward from a vertical rim plate at the tank wall to a circular gas dome at the center of the tank. This slope helps convey the methane gas produced by the anaerobic digestion process, towards the center for collection and removal.

The rigid floating digester covers are limited in their ability to store methane gas for heating the sludge contents. They are also prone to tipping and binding within the digester tank which can not only damage the cover, but also, the internal piping within the tank.

A key problem with these traditional rigid floating covers is simply their advanced age. After about 30 years of being in the corrosive environment of organic acids and methane/sulfur gases, the carbon steel used in the covers construction is no longer structurally sound. The corrosion is typically so pervasive that rehabilitation is not cost effective. Hence, the owner is left with the replacement option only, when considering a structurally deficient cover. Fortunately, we are no longer bound by the old rigid steel cover design, as material technology has advanced in recent years to enable a new flexible membrane design to emerge.

The Dystor system is a gas holder design that uses a dome-shaped, engineered membrane system to store methane gas, provide for sludge storage and prevent odors.

The system includes two durable membranes. The outer membrane is cable restrained and remains inflated in a fixed position. An inner membrane moves freely as it stores or releases gas generated from the anaerobic digestion process.

An air handling system maintains a preset operating pressure between the two membranes. This keeps the outer membrane inflated, while exerting a constant pressure on the stored gas regardless of inner membrane position.

Operating pressure is easily varied within the design range without adding or removing ballast.

Membranes are sealed tight to the digestion tank's wall, preventing odors from escaping. In operation, as methane gas is withdrawn from the gas storage chamber, a fan supplies air to the air chamber. This expands the air chamber so it displaces the methane gas.

As gas is added, an adjustable pressure relief valve equalizes the pressure in the two chambers by venting the air chamber and allowing the gas chamber to expand.

There are two configurations of the gas holder domes available, depending upon location and need:

The Free-Membrane Cover (Photo 1) is designed for secondary digesters or anaerobic sludge storage tanks. The free-membrane cover provides the greatest volume of air chamber displacement for gas storage and sludge variation. At low sludge levels, the inner membrane extends down into the tank, essentially as a mirror image of the outer membrane.

The Membrane Cover with Support Dome (Photo 2) is intended for either a primary or secondary digester with piping, mixing cannons or other equipment that could interfere with the travel of the inner membrane. The inner membrane is permanently fixed at the top, so it cannot extend very far into the tank when the system is out of service and deflated.

The Village of Springville wastewater treatment plant's primary digester will be retro-fitted for a new flexible gas holder cover with support dome this summer. It will increase the sludge storage capacity of the unit by 50 percent and increase the gas holding storage volume by 1000 percent, almost eliminating the need to subsidize the boiler with expensive natural gas. The installation of this cover in the Village of Springville will be the first of it kind in New York State.

GLOBAL POSITION SYSTEM REVISITED

by James A. Nearhood, PLS

Many improvements have been made in the private citizen's use of the NAVSTAR satellite system since my last article on this subject in July of 1993. The small hand held units for our navigation on land or sea not only tell us our present position but also the bearing and distance to almost any number of previously entered points (how to get back to camp) the path you took that day and the way home is also shown on the LCD screen.

The surveyors use of these radio signals from space has also seen changes. For one thing we now have 24 USA satellites dedicated to military positional and navigation use, that's twice as many as we had in 1993. Also we can use, with the proper computer software, the Russian "Glasnoss" satellites which again doubles the stations available.

Improved computer software has helped compensate for some of the inherent clocks, user's receiver clocks, ionosphere and tropospheric refraction, multipath reflection and the deliberate degradation/alteration of the signals by the military. The hardware of the receivers and radio links between ground stations have shrunk in size, but not in cost. Where before we had to use two tripods with bulky equipment and antenna or backpacks, we now can use a hand held computer with a small receiver and antenna mounted on one pole. This makes it more mobile, thus much more useful and less time consuming.

The use of GPS by local surveyors is increasing in the field of global information system mapping and large surveys with unobstructed land conditions. The traditional survey crew is still the method of choice for local residential and rural surveys.

BRIDGE PAINTING - WHY, WHEN, HOW, HOW MUCH ($)?

by: Roy R. Pedersen, P.E.

In our daily travels to work, school, and shopping, we cross bridges many times. Many of these bridges have steel beams that are exposed to the elements. Most of us can remember a bridge that we have driven on or under whose steel members were visibly rusting. It is up to our elected and appointed officials, and our Local, State and Federal agencies to decide if these bridges get painted, and when.

WHY: This is the easiest question to answer. By painting a rusting steel bridge, it can live longer. Rust eats steel. If allowed to go unchecked, the beams could literally be eaten through to the point where the bridge would collapse under the weight of its deck. If the steel does not rust, a life expectancy of 100 years is reasonable.

Perhaps a better question is "why not?" There are various reasons that a bridge may not be painted. If a bridge is scheduled for replacement in the near future its owner may not realize any financial benefit from the expense of painting.

Many bridges are now constructed of "weathering steel". This is a higher strength steel that has the capability to resist rust. Once the initial film of rust develops, it seals out moisture and stops rusting. (In certain hostile environments however, this steel can continue to rust.) So, if you see a relatively new bridge that is rusty looking - it may be ok if it was built out of weathering steel.

Another reason why not to paint a bridge, is money. It can be very expensive to repaint a bridge. This may require special financial planning to be able to afford the job. If the existing paint has lead in it, special, expensive procedures are required to contain the lead. If the bridge is over a stream, it gets more expensive. There are instances where some bridges might be closed instead of going to the expense of painting them.

WHEN: If you have decided to paint a bridge then the decision is - when? This is an economics trade off. It is desirable to get your moneys worth from the initial paint while not waiting so long that the cost of repainting becomes very high.

We have seen unpainted bridges deteriorate to the point of requiring a lower load posting every year. On these, the owners waited too long. In defense of these owners, they work with limited budgets and sometimes painting a bridge isn't the hottest fire they need to extinguish.

HOW: There are many types of paint products to choose from, including Alkyds, Urethanes, Epoxies, rust inhibiting paints such as Rust Oleum and moisture curing paints which can be applied in damp or cold conditions.

There are also various ways to prepare the surface to be painted. Sand blasting is very common, but it leaves the sand to be cleaned up. If lead paint is being removed, the sand is contaminated and must be disposed of as hazardous waste.

There are vacuum blasters which vacuum the sand into canisters for easier clean up. Another method is high pressure water cleaning. This uses no sand and therefore is easier to clean up after.

The Steel Structures Painting Council has prepared a manual to recommend the type of surface preparation method best for different applications.

HOW MUCH: The cost to paint a structure will vary with size, location, condition, and type of paint on the steel.

Nearby Allegany County, New York paints their own bridges. Their goal is to revisit each bridge on a five year interval. This keeps the paint from deteriorating too badly, thereby making the repainting job easier. The estimated annual cost per bridge is easily less than $500. The State Highway Departments spend more on painting projects, but expect longer intervals between paintings - say 15-20 years.

Considering the cost to build new bridges, they are definitely worth painting if they are still structurally and functionally serving the transportation need they were originally built to satisfy.

CLASS HP CONCRETE

One of the major reasons for a bridge rehabilitation project being undertaken is deterioration of the concrete bridge deck. The top layer of reinforcement corrodes and causes spalling of the deck surface and a rough ride for travelers. Some of the more recent modifications which were made to delay the corrosion of the reinforcement were the use of epoxy coated reinforcement and placement of the reinforcement further below the surface. Significant reductions in corrosion could be realized if cracking of the deck and permeability of the concrete could be reduced thereby slowing the attack on the steel by deicing salt.

Since 1997, New York State DOT has required the use of high performance (Class HP) concrete in all bridge decks. Class HP concrete is a modification of Class H concrete which substitutes 20% Class F Fly Ash and 6% Microsilica for cement. The water/cement ratio is .40. The use of the fly ash and microsilica has reduced permeability by approximately 70%. The lower water/cement ratio has helped reduce cracking by approximately 98%. By using the Class HP concrete the life of concrete bridge decks should be extended from 20 years to 50 years.

Although the cost is higher (10%-20%) than Class H Concrete the extended service life will more than recoup the cost of the investment. If you're with a local municipality which is undertaking a bridge project you should seriously consider Class HP Concrete.