How to Design an ATU/drip System

The first thing that must be considered when designing an aerobic treatment unit (ATU)/drip irrigation system is what treatment unit product to use. There are several products available in the Florida market. The product you choose may be directly related to the application. For instance, a restaurant would be better served with a unit that uses a fixed film media or a unit that is designed for high strength wastes. A single family residence would not require this type of treatment unit.

Another consideration is what construction material is best for the site; concrete or fiberglass? Some sites are best suited for fiberglass units for no other reason than access. Large crane trucks are required to set concrete tanks and there may not be enough room for the truck to maneuver. An example of this would be a site where the tanks need to be placed in the back of the house and adjacent lots are developed. Fiberglass units are easily transported and moved into place by the installer using their backhoe or Bobcat. Some installers prefer fiberglass units simply because they don’t have to rely on someone else to set the tanks.

The next step is to determine the estimated sewage flow. Once you have established the estimated sewage flow, the treatment unit size should be selected. It is vitally important to understand that consideration must be given to the strength of waste that is anticipated. Sizing the unit by estimated flow alone may not be enough. The treatment unit size must be sufficient for the biological load of the effluent.

The following is a brief list of the information you need to determine in order to design the system:

1. estimated sewage flow
2. soil texture loading rate
3. square feet of application area
4. linear feet of tubing
5. number of drip emitters
6. drip rate
7. system operating pressure & operating pressure at the drip emitter
8. gallons per minute being disposed
9. number of doses per day
10. time between doses
11. total run time per day
12. recirculation rate
13. scour velocity

The system operating pressure is required in order to determine the total dynamic head (TDH). TDH is comprised of three components; static head, friction head (friction loss) and operating head. Once you have determined the TDH, you can determine the gallons per minute that your pump will generate. This information is required in order to determine the scour velocity that will be achieved. Florida code requires 2 ft/sec.

Designing a drip system is a step by step process but your design won’t necessarily follow the items above in order. In fact, you will need to determine the square feet of dripfield and lay your system out on the site plan before you can determine TDH. Be aware that you must also consider the loss through filters, flow meters, etc. in order to arrive at an accurate number.

If space is a real issue, you may spend a lot of time trying to figure out how and where you can fit all of the drip line meeting all appropriate setbacks. You may need to utilize a greater drainfield reduction in order to fit a system on a site. Reductions up to 40% can be applied but require performance base treatment systems (PBTS).

Quick Tips

All ATU/drip systems should have a trash tank. Some ATU manufacturers have minimum recommendations. A good rule of thumb is that the trash tank should be sized at ½ to 1 day of sewage flow, although it depends on the specific application.

If at all possible, conduct the site work yourself. Many engineers/designers rely on someone else to conduct the site and soils work. A good design starts with the soil. If you are concerned about making good soil calls, get trained. DOH offers a soils course specific to onsite work. The Florida Association of Environmental Soil Scientists (www.faess.org) offers a hydric soils workshop annually. This is an excellent course to expose you to the indicators that are present in the soil as a result of seasonal inundation/saturation. Even if you are already doing the soils work yourself, attending the hydric soils workshop will fine tune your skills.

Understanding the site conditions (available space, slope, access, etc.) goes a long way in system design. It should be your goal to design a system that for the most part gets installed the same way you laid it out on the site plan.

Work closely with the installer. The more you understand about the installation process and how systems actually go in, the better designer you will be. Designs should be installer friendly.

Your goal should not only be to design a good system. Your goal should also be to submit a design that contains ALL the required information. This will ensure a quick turnaround through DOH which will make your client happy. Happy clients results in more referrals!

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