SUBSURFACE DRIP DISPERSAL OF PRIMARY TREATED EFFLUENT

2/27/98

The use of WASTEFLOW® for subsurface drip irrigation dispersal of secondary treated effluent is well known and well proven in more than 1,000 systems both large and small. Ayers and Associates under a grant funded by the EPA, have been conducting experiments in the Florida Keys by taking septic tank effluent from a prison and passing it through two Geoflow VORTEX™ filters and then into a WASTEFLOW subsurface drip system in sand and two artificial mediums. These experiments have been very successful both from the point of view of the excellent performance of the filters and the WASTEFLOW but also from the point of view of absorption of nitrates and phosphates. Caution should be exercised in extrapolating the experience gained in sand to heavier soils. The purpose of this paper is to combine the knowledge that we have at this time to establish guidelines which are conservative enough to ensure safe reliable dispersal of primary treated effluent at an economic cost.

Figs. 1 shows the layout of a typical system. We recommend using the WASTEFLOW Manager™ for control, monitoring with auto-dialer, and data logging, but lower cost simple controllers can be used.

Referring to Figure 1:

1. Ensure that the septic tank is adequate in size and design for the designed flow.

2. Use an in–tank filter in the septic tank.

3. The VORTEX filter operates alternatively either supplying effluent to the drip irrigation field or being forward washed through the centrifuge. Ensure that the flow through the filter is within the optimal design flow for the size of filter selected – see figure 2: VORTEX filter design parameters.

A critical factor is the sizing and design of the drip dispersal field.

Drip dispersal is a point source application and failure comes about by overloading the point with BOD and forming an anaerobic bacterial mat (schmutzdekke). Therefor we need to reduce the bacterial load per emitter down to the levels which we have found by experience to be failure proof when applying secondary treated effluent. If we use 12" x 12" spacing of drippers and driplines we have immediately increased the number of drippers by four-fold over the known configuration of 24" by 24" spacing. If we then increase the area by doubling it we are up to 8 times the number of drippers as compared with the number of emitters we are experienced in using when applying secondary treated effluent. Table 2 below sets out these standards.

Dripper selection: To give the aerobic soil bacteria maximum time to absorb the anaerobic bacteria in the effluent it is useful to use the 1Ž2 gallon per hour emitter. For effluent the 1Ž2 gph flow rate is only available in pressure compensating WASTEFLOW.

To summarize the above arguments for this application we recommend WFPC16–2–12.

Control and Monitoring:

The use of septic tank effluent obviously carries considerably higher risk to both health and environment than the use of secondary treated effluent. Geoflow offers a WASTEFLOW Manager which will control and continuously monitor the septic tank, the pump chamber, the filters and the subsurface drip dispersal fields. The Manager will automatically dial to up to six 'phone numbers in the event of any failure. The Manager can be accessed and both operated and re-programmed over a telephone line. The Manager will hold one year's data in the data log. We strongly urge that larger systems using septic effluent should consider the technology of the WASTEFLOW Manager to ensure safe and effective operation.


Figure 1: High BOD effluent filtration

WASTEFLOW MANAGER is the preferred controller and monitor for larger systems using a flowmeter to continuously monitor flows.

For small systems which are not monitored the preferred controller is the GEO-1 or GEO-2

Typical Program:

24 cycles each hour per day. Each cycle 4 minutes.

At the end of each cycle flush the filter (open valve B) for one minute.

Flush the system (open valve A) every 100 cycles.

 

Design Information:
  Minimum Maximum
Length of WASTEFLOW 250' 10,000'
Flow at each valve 4 gpm 21 gpm
Dripline length per valve    
Classic 24" x 1 gph 400' 2,250'
Classic 12" x 1 gph 200' 1,000'
P/C 24" x 1 gph 500' 2,500'
P/C 12" x 1/2 gph 500' 2,500'

 

Cleaning the VORTEX filter

Scrubbing very fine particles with a brush does no good, and filters "cleaned" in this way tend to plug very quickly.

Clean with a pressure hose from the outside of the mesh cartridge inwards to the middle. Also because we are dealing with high strength effluent a soaking in chlorine will help. The screen is stainless steel and this will not have any adverse effects. Use 50% Chlorox with 50% water. Chlorox is 7% chlorine.

 

SUGGESTED   APPLICATION   RATES          
                   
    Typical treatment:   Secondary
Lagoon
       
    Maximum >Minimum BOD < 30 mg/l BOD < 60 mg/l BOD < 90 mg/l   BOD < 150 mg/l BOD < 180 mg/l
Texture Approx. Application Area in sq. ft Min. # Min. # Min. #   Min. # Min. #
Group Perc rate GPD/Sq.Ft. / 100 gpd drippers/sq.ft. drippers/sq.ft. drippers/sq.ft.   drippers/sq.ft. drippers/sq.ft.
I Coarse sand >5 1.00 100
1.00
1.00
1.00
1.25
1.50
I Fine sand
5 - 10 0.80 125 0.50 0.50 0.75 1.25 1.50
II Sandy loam 10 - 20
0.60
167
0.25
0.50
0.75
 
1.25
1.50
II Loam
<20 - 30
0.50
200
0.25
0.50
0.75
 
1.25
1.50
III Clay loan
30 - 45
0.40
250
0.25
0.50
0.75
 
1.25
1.50
III Silty clay loam
45 - 60
0.30
333
0.25
0.50
0.75
 
1.25
1.50
IV Sandy clay
60 - 90
0.15
667
0.25
0.50
0.75
 
1.25
1.50
IV Silty clay
90 - 120
0.10
1000
0.50
1.00
1.00
 
1.25
1.50
IV Clay
>120
0.05
2000 1.00 1.00 1.00
 
1.25
1.50
                   
                   
   
Typical treatment:
 
Filtered Septic tank
         
   
Maximum
Minimum
BOD < 120 mg/l
BOD < 150 mg/l
BOD < 180 mg/l
     
Texture
Approx.
Application
Area in sq. ft
Min. #
Min. #
<
Min. #
     
Group
Perc rate
GPD/Sq.Ft.
/ 100 gpd
drippers/sq.ft.
drippers/sq.ft.
drippers/sq.ft.
     
I Coarse sand
>5
1.00
100
1.00
1.25
1.50
     
I Fine sand
5 - 10
0.80
125
1.00
1.25
1.50
     
II Sandy loam
10 - 20
0.60
167
1.00
1.25
1.50
     
II Loam
20 - 30
0.50
200
1.00
1.25
1.50
     
III Clay loam
30 - 45
0.40
250
1.00
1.25
1.50
     
III Silty clay loam
45 - 60
0.30
333
1.00
1.25
1.50
     
IV Sandy clay
60 - 90
0.15
667
1.00
1.25
1.50      
>IV Silty clay
90 - 120
0.10
1000
1.00
1.25
1.50
   
u>IV Clay
>120
0.05
2000
1.00
1.25
1.50
     
Table 2: SUGGESTED APPLICATION RATES OF EFFLUENT