Anatomy Of A Boat Wash Pad

A wash pad should be considered an integral part of a wash rack operation. this seemingly non purposeful object is typically disregarded as a root spot where to park the device you are about to wash. This is the furthest from the truth when considering filtration capacity of the surface area and the potential means by which the wash water management is initially generated. This document is designed to provide the basic information about the wash pad, its design characteristics and the integration of this main component to any wash water operation.

The wash pad shown here is considered a properly designed wash pad. the following bullet point describe why.

  • The grade of the pad is designed to move the water to one corner of the pad using a shallow slope, 1/8″ per foot, which allows solids to remain on the wash pad and not be swept off from aggressive flow. The wash pad itself should be used as a very large filter. The surface area and gradual slope filters many solids so that the water treatment equipment will not be overwhelmed with excess solids
  • The wash pad is designed with a Rain Diverter that will remove storm water off the pad without entering the water treatment equipment. The design incorporated here shown in figure 2 uses a sliding gate that when closed blocks the path of the water to the trench and the grade of the wash pad then allows the water to leave the pad at the corner. The wash pad will need to be cleaned after each use so that when the wash pad is set up to receive rain water, the rain water will not flush previous wash contaminates off the pad to the storm drain. This practice is described in the Florida Department Of Environmental Protections best management practices for wash water recycle operations. See figure 4. Similar EPA instructions are written for each state.
  • The trench shown in this wash pad design becomes the second phase of wash pad filtration by again generating a shallow grade, 1/8″ per foot slope, to allow solids to settle in the trench before reaching the water treatment equipment. The trench is designed so that no water is allowed to collect in the trench itself. The trench can also be fitted with removable weirs set in the bottom of the trench to further enhance the filtration by creating treacherous paths where solids will tend to accumulate. the weirs can simply be 2″ angle iron that can easily be removed to allow a square nose shovel to be used for collected solids removal. A shop vacuum also works well to remove solids from the trench.
  • An in ground separation/settling pit can be used in place of the trench to allow for oil/water separation through baffling or solids settling through the use of water volumetric. Using water volumetrics for settling increases the potential for BOD generation through reduced dissolved oxygen in the volume of water in the pit. Using water volumetrics also increases the amount of wet sludge that would eventually need to be hauled off site. Allowing solids to remain in a settling tank for long periods of time increases the contaminant levels in the sludge as more solids are added. Increased contaminant levels increases the amount paid to have the solids hauled off site. Solids that settler on a properly designed designed wash pad contain less contaminants and would be easier to discard.
  • A sump at the end of the trench is used to collect and transfer the water treatment equipment. the pump in the sump should be elevated off the bottom of the sump to allow for some solids to settle below the suction of the pump. The sump should regularly be cleaned to remove collected solids before reaching the sump suction.

The rain gate can be designed in a variety of manners from simple gates like the one shown in figure 2. They can also be elaborate using ball valves and automated gate type valves that that can be controlled simply by using a flow switch on the wash water recycling system indicating that the wash pad is in use. The device ensures that no rain water will enter the water treatment equipment by re-directing the storm water safely off the wash pad. It is important to ensure that the wash pad is cleaned after wash use to reduce the contaminants that could be flushed off the pad during a rain event.

Figure 3 is a view of the trench and sump configuration as designed in figure 1. The trench area is elevated above the wash pad at least 4 inches to allow for the use of the rain gate. the trench and sump can have either a solid cover or a grating depending upo the area and choice of the client. The trench should be wide enough to allow the use of a square nose shovel to clean to accumulate solids from the trench. 2″ angle iron weirs can be set across the bottom of the trench a few feet apart to create a treacherous path to allow solids to settle increasing filtration. the sump should have a trash pump sized properly to allow the collected water to be sent to the water treatment equipment. De-tune the pump by installing a tee and ball valve in the line to decant flow to set the proper GPM going to the water treatment equipment.


Figure 16: The rain falling onto this wash pad can be diverted to the storm water sewer via a removable rain water dam. This prevents the recycle system from overflowing due to excess volume. The wash pad must be thoroughly cleaned before diverting the rainfall off the pad and into the storm sewer.



Figure 17: This facility does not have a roof or other cover the wash area. As a result, the storm water occasionally creates an overflow of waste water into the storm water pond adjacent to the system. This is not a desirable solution.

if a wash pad can not be covered with a roof for various reasons (including cost) another way to prevent stormwater from overflowing the system, is to install a storm water diversion valve. this allows the system to discharge uncontaminated stormwater runoff from the wash pad to the appropriate storm water outfall when the pad is not in use.

Designs To AVOID

It has been the conventional wisdom to use large column tanks of water to settle solids as a pre-treatment. This process obviously works to settle solids and has been used for many years. The issue with this process is the solids accumulate in the pit or tank, the organic;s that are intermixed begin to degrade and absorb dissolved oxygen creating a septic condition. As water is circulated the septic water is mixed throughout the entire system decreasing the overall water quality. Another disadvantage of large settling pits is that the solids that accumulate generate high costs when they are removed from the pits and hauled off site. typically these pits are left without attention and the solids to build to a point to where the use of volumetric settling no longer exists. The solids that that accumulate also give off increasing contaminants being dissolved by water they sit in which further reduces water quality in the entire system. To counter this an operator will need to add additional oxidation to combat the increasing BOD. Usually the addition of oxidation is done in large quantities added one at a time where there may be a spike in the oxidation potential but is quickly absorbed by the strong septic conditions. A wash pad should remove as much of the solids as possible prior to coming in contact with volumes of water. If this design is needed or unavoidable then it must be stressed that the solids will need to be evacuated often in order to not impact the entire system.

The Center Sump Design

Center sump designs present a few challenges. One of them is described above where the volume of water that accumulates in the center pump can increase the overall contaminant level in the entire system from the degradation of the accumulated solids that will remain in this design. Usually this collection area is not circulated and during the non operational periods which could be lengthy creates an environment suitable for anaerobic bacteria to flourish. When the sump area is used in a subsequent wash the septic volume is transferred into the water that has been filtered and oxidized reversing the treatment that has betaken place. Another issue with this design is is the reduced ability to divert rain water from the water treatment system. The issue with rain water diversion using this design is that the rain water diversion technology is typically a valve system that sends rain water that collects in the same pump[ as wash water out of the collection sump to a storm drain or swale. The rain water that is now contaminated with wash water is now in turn contaminating the storm drain or swale creating an illegal discharge. A method used to combat this has been to send the first flush from the wash pad to the water treatment equipment by using a rain detection device and timer to first pump the contaminated rain water to the treatment and then change the valves position and send the subsequent volume to the storm drain or swale. A program such as this becomes difficult to manage since rain events can be light or heavy where as the timing device has no control over the volume of water that is first sent to the water treatment equipment creating overflow conditions.

Aggressive Slopes: In order to use the wash pad as a very large filter, the grade or slope used to direct water to a collection area must be minimal to allow the solids to settle and the ones that do can be flushed through the trench rendering the design useless.

Favorable Designs Styles

Draining solids from water treatment equipment: all wash pad designs should have a means to drain the solids that will accumulate in the water treatment equipment tank and filter housings back to the wash pad or to the beginning of the trench. This will allow frequent solids relief from the water treatment equipment and send it where it can be easily removed. Some designs send the draining to the first part of the trench to get the full length of the trench to settle the solids and some designs send the draining to an opposite corner of the pad and use the wash pad itself to capture the solids from the draining.

Circulating The Water: When looking at at the design of the wash pad, circulation is a required step. the water in a water treatment system needs to be circulated with volumes that may be ion wash pad designs to ensure they are included in the the oxidation potential of the system. If the wash pad uses a volume of water for separating or settling, ensure there is a bleed back from the water treatment equipment to this volume of water to generate full system circulation. This process will ensure that during the non operational periods there is no standing water. When looking at this ensure you do not create a situation where rain water diversion will empty the water treatment system from the bleed back operation.

Size Considerations: Wash pad design must complement the operation and assist in the settling and removal of solids. Ensure the wash pad is large enough for the equipment being washed and that there size allows for solids removal by not having to park the vehicle to be washed right next to the collection area. This mistake will limit the wash pads ability to filter the solids. When designing a wash pad the bigger the better. Larger pads also assist in evaporation of the water where fresh water rinses are allowed to help the overall water quality of the system. Evaporation of the wash water on the pad removes contaminants from the system and collects them where they can be easily removed, on the pad.

Sanitary Sewer: If discharge to a sanitary sewer is available, it could help maintain the overall water quality by overflowing rinse water volumes to this entity. Regulations allow for a 20% discharge to sanitary sewer but the proper permits need to be acquired and testing ma be required. This allows for more liberal rinsing.