WasteWater

In addition to the waste water papers listed in DOWNLOADS, the following interesting paper was given at a "Clean Water Safe Harbor" meeting.

SHORT INTRODUCTION

Norm Morris, president of Delta Pollution Control, Inc., has been building metals removal treatment systems since 1973. The company began by building metals removal treatment systems for the plating and printed circuit board industries. For the past four years Norm has been working with boat yards and ship yards in the Pacific Northwest building metals removal systems from waste water generated from pressure washing and blasting operations.

When developing a plan to treat waste water generated from water blasting or wet abrasive blasting operations there are four basic things that must be closely evaluated.

If the application for instance is a dry dock operation that is cleaning ship hulls or removing paint from ship hulls, the first thing that has to be done is to come up with a method of capturing the wastewater being generated. Large dry docks are normally sealed with steel plate with a crown in the middle and trenches on each sides. The dock master is asked to lift the dock so there is a slight slope to the dock so all the water will run to the shore end of the dock. Collection sumps are placed at the end of the dry docks which are plumbed together to form a common sump. A large drain valve is placed in the sump to drain the dock when lifted and for rain diversion.

A consideration that now has to take place is negotiating with local state authorities as to rain water collection. Some states require that all rainwater falling on the dock while a ship is up must be collected and treated. Other states have accepted that the first '/2" of rainfall on the dock is sufficient to flush the dock and that the rest of the rainfall may be diverted. This consideration is very important in your next step which is sizing your pumps and holding tank.

Typically a pump is placed in the sump to handle the waste water being generated at normal washing rates. If a rain event occurs the primary pump will fall behind and the sump will rise until a second pump of greater velocity comes on. They will pump simultaneously to keep up with the rain event. As the rain event slows the secondary pump will shut down and the primary will keep up. These pumps are sized depending on how much water is generated during hull blasting and the surface area of the dock which determines how much rain water will be collected. Another factor is how hard it rains. Out on the west coast it typically rains a lot but not very hard. On the east coast there are very rapid showers which means it comes fast for shorter duration's.

Now comes sizing of the holding tank used prior to treatment. You must calculate the surface area of your dry dock and see how much water will be generated in a rain event coupled with your pump usage. The holding tank must be large enough to conservatively handle at lease two rain events plus pump water from blasting.

These are some rough calculations that you might use to get an idea of sizing. If all the rain water must be captured you have to do some serious rain studies in your area to calculate how much you might generate and how fast to treat it. You should look at annual rain fall data to see how much rain fall occurs in what months, etc. to aid in holding tank sizing and transfer pump sizing.

It is also a good idea to keep the holding tank agitated to keep solids from building. This minimizes going in with shovels or scuba gear to remove the build up that will eventually occur. The treatment system should be designed for solids removal instead of pre-settling solids in a settling tank or lagoon that has to be eventually mucked out.

One of the major costs involved in a treatment system is how low the discharge contaminate numbers are that must be met. You now have to look for options for discharging the treated waste water. Each of these options will have different discharge criteria. Some of the options typically are:

POTW's treat the water again on the way to its discharge point and typically have a less stringent limit to meet. For instance copper might be 2 -2.5 ppm (parts per million) and the same or higher for zinc. The greatest advantage is having the ability to discharge the treated rainwater which builds in the system. This treatment system would be less expensive because it wouldn't have to be as sophisticated.

Discharge to sprinklers is sometimes an option in warmer climates. State regulators are typically fond of applications where the water can be reused rather than discharged. These limits are typically tighter than POTW discharge, but still sometimes are as high as 1 ppm of copper which is not as difficult as discharging to the river or sound.

Some locations have storm water retention ponds that the surface water from their site runs into and seeps into the ground. These limits are typically similar to the standards of the sprinklers, but can be more stringent on the discharge requirements if the regulator deems so.

This standard typically cannot be met by the water coming out of your washdown hose because it passes through copper or galvanized pipes. This limit is extremely difficult to meet and would take a much more sophisticated treatment system. System cost coupled with technical expertise to run the system really does not make it a practical option. However, it has been done, by being allowed to utilize a mixing zone in the river prior to river discharge. This must be worked out with the local regulation authorities.

When first looking at it this seems the best option because you don't have to get permits or worry about meeting discharge limits. However in reality this is the most difficult of all. Ultra High Pumps require water of very high purity unless you like buying spare parts. In a closed loop system there are salts for instance, that accumulate in the water such as salt (sodium) which comes off the hull in the organic growth. This continues to build causing problems. The biggest problem however is rainwater. You will experience evaporation in the system naturally by the warm climates and the heat from the pumps, but rainwater can be significant. A separate evaporation system will have to be installed after the solids removal treatment system to keep the system from building water volumes. If the solids are not removed first, the evaporator will clog with solids lowering heat transfer efficiency.

After determining how much water will be generated and what you plan to do with it after treatment, you can look at the treatment system. System costs are determined by volume to be treated and how difficult the discharge standards are that must be met.

When looking at treatment systems keep a few things in mind. You are primarily looking at fairly high solids removal of very fine particulate. There will be some metals such as copper, zinc, maybe lead that will actually be dissolved in the water as well. If you buy a system that removes particulate for instance it will not remove the dissolved metals. There may be some oils that get in the water from heavy equipment on the dock or incidental bilge water that may have to be addressed. Free oil will float on your holding tank but emulsified oil will not.

For instance don't buy an oil water separator. There isn't much if any oil in the waste water and it will just plug with solids. Since the waste water stream will have high solids, inline filters such as cartridge or large sand beds will plug very quickly. You want a system to remove solids and also draw dissolved metals from the water as well.

The most common technology that works well in this area, is the least sophisticated to run, and is the most cost effective to purchase and operate, is a chemical destruct and precipitation system. The wastewater is drawn from the holding tank at the prescribed flow rate determined and sent to a retention tank with constant agitation. A chemical is added to the stream to draw the dissolved metals out of solution to form fine particles in the stream with all the undissolved particulate already present. This tank overflows into another smaller tank where a second chemical is added that has a charge opposite the charge on the fine particulate. This causes all the fine particulate to gather together into a large flake with a much heavier specific gravity causing it to fall rapidly when agitation stops. The stream now flows to a settler, usually a clarifier, where all the particulate falls to the bottom and the clear water migrates to the top. The sludge forming at the bottom is drawn off and pressed under 90 psi into firm cakes for disposal. The clarified top water could pass to a POTW. For more difficult applications it typically would pass though a gravity sand filter to capture any extremely fine particles. The sand filter would not be backwashed often because 99% of the solids have already been removed from the waste stream. If extremely low limits have to be met an ion exchange unit similar to a water softener would be added after the sand bed filter to go after very small amounts of dissolved metals if your trying to meet river standards.

This system can be made to work automatically at flow rates from 10 gal/min to 200 gal/min or more. In small boatyards or on site jobs such as blasting paint off buildings, bridges, or roads batch treatment is often utilized in batch tanks of 350 to 1500 gallon batches. Sludge is dealt with in hanging bags, false bottom dumpsters, or filter presses.

2. Be sure you know what you are going to be able to do with the treated water before buying a treatment system.

3. Buy a treatment system that is big enough to treat the amount of waste water you will generate, and be sure its deign will remove all contaminates you need to have removed for discharge.