' r /f/" ugust 1986
Irrigation of Urban
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences
University of Florida, Gainesville / John T. Woestel lean for Extension
L.B. Baldwin and D.A. Comer MAR 181987
University of Florida
Table of Contents
Introduction .................... .......................... 1
Wastewater Treatment Processes ................................... 1
Primary Treatment ............... ........................ 1
Secondary Treatment .............. ....... ..... ............. 2
Disinfection ................... .... .................... 2
Sludge Processing.................... ..... .............. 2
Characteristics of Secondary Effluent and Sludge .................... 3
Advanced Wastewater Treatment ............... ... ....... .... 3
Regulations for Landspreading Treated Effluent .......................... 4
Landspreading Systems ................... ............... 4
Required Treatm ent ........................................... 4
Storage Facilities ............................ ................ 5
Buffer Zones ......................... .................... 5
Groundwater Monitoring ............... ... ................ 5
Additional Requirements ............... ...... ............... 5
Economic Implications ofLandspreading Effluent ......................... 5
Sewage Treatment Costs .................... .. ............ 7
Value of Sewage Effluent for Urban Irrigation ....................... 7
Cost of Delivering Effluent .................. ................ 9
Summary of Benefits and Costs .................... .......... 9
Summary and Conclusions .................. .. .............. 10
UTILIZING TREATED SEWAGE
FOR IRRIGATION OF URBAN LANDSCAPES
L.B. Baldwin and D.A. Comer*
The use of treated sewage for irrigation is increasing in Florida because this disposal
alternative is attractive economically and because the practice conserves water. The
/ feasibility of irrigating with effluent from any given sewage treatment plant depends
on many factors that require careful analysis by both the utility agency wishing to
dispose of effluent and the potential users of the effluent.
Guidelines for effluent use have been developed by the Department of Environmen-
tal Regulation (DER), and a treatment method for pathogen control, tested at St.
Petersburg, has been approved by health department officials. This publication will
present some aspects of the economic and regulatory constraints that must be con-
sidered in analyzing a potential landspreading project.
Although the water management districts in Florida strongly encourage the use of
sewage effluent to reduce withdrawals from water supplies, the adoption of this prac-
tice usually depends on whether it can reduce the cost of sewage disposal. Because
urban areas are the producers of large amounts of sewage, urban landscape irrigation
is a promising potential use for effluent that has been treated for pathogen control. A
brief discussion of sewage treatment, the characteristics of the treated effluent, and the
costs and potential benefits will aid in understanding the economic issues of providing
acceptable effluent to urban users, including golf course managers.
Wastewater Treatment Processes
Municipal wastewater must be collected when housing and commerce become too
concentrated to allow for safe on-site disposal. When collected, municipal sewage is a
continuous but variable stream of potentially harmful material that should not be
discharged untreated into the environment. A population of 10,000 will generate about
1 million gallons of sewage each day (1 mgd), or about 100 gallons per person.
In raw form, sewage contains only a small amount of solids, usually around .05 per-
cent; however, these solids include pathogenic organisms, putrescible organic matter,
plant nutrients and sometimes toxic substances. The essential purpose of treatment is
to neutralize the potentially harmful aspects of sewage, and this is done by removing
some solids, converting some substances to more inert forms, and disinfecting the
effluent. Several processes have been developed to achieve these objectives. The
sewage is passed from process to process with controlled changes occurring at each step.
This section will describe some typical processes in the order they are utilized in sewage
SThis first stage of treatment is designed to remove anything that can be screened, set-
tled, or floated out of the waste stream. Sewage from residential areas contains a fairly
predictable level of dissolved and finely divided organic solids; however, it can also
include a variety of objects small enough to flush down a toilet. Many articles that are
disposed of in this manner, such as cigarette filters, plastic articles, rags, etc., are not
*Associate Professor, Agricultural Engineering, and Associate Professor, Food
and Resource Economics, Institute of Food and Agricultural Sciences, University
of Florida, Gainesville, FL 32611.
easily biodegradable. In addition, some sewer systems pick up roof and street drainage,
which carries sand, fine gravel and a wide variety of larger articles. Screens, grit
chambers, grinders, and sedimentation tanks are employed to remove solids prior to
SThe major objectives of secondary treatment are to satisfy the biochemical oxygen
demand (BOD) of material dissolved in the waste stream and to provide further
removal of suspended solids. Most secondary processes are biological, requiring an
environment suitable for sustained biological digestion of organic materials in the
waste. Bacteria, protozoa, rotifiers, fungi, and algae are active in the process. Organic
material is digested by these organisms as food, thus converting the material into car-
bon dioxide, water, and cellular material for growth and reproduction. Adequate
oxygen and a favorable temperature range are required for this process to take place.
Several methods have been developed to achieve the conditions needed for effective
biological waste treatment. The most common secondary treatment methods used in
Florida are mechanical aeration of a mixture of recycled sludge and wastewater, and
trickling wastewater over biological filter beds.
The last treatment step prior to release of effluent is the addition of a disinfectant for
the purpose of killing pathogenic bacteria and viruses. Chlorine is the most widely used
disinfectant for this purpose, although ozone, a more costly substance, is receiving
attention due to concerns over some residual compounds of chlorine and organic matter.
Both chlorine and ozone oxidize organic present in the treated wastewater, and unless
supplied in sufficient quantities, the disinfectants may be "used up" for oxidation
before all pathogens are destroyed. This is a particular concern with viruses, which
tend to be strongly associated with organic particles, thus being protected until all the
particles are oxidized.
Following disinfection, the treated effluent is released from the plant for stream
discharge, landspreading, or some other disposal. Short- or long-term storage in ponds
may allow for some further changes in effluent quality and volume through biological
actions, seepage, or evaporation.
The materials that settle and float out of the wastewater during primary and second-
ary treatments are called sludge, which is very dilute up to 97 percent water when
drawn from the sedimentation tanks (clarifiers). The quantities of sludge involved are
significant, totalling around 20,000 gallons per million gallons of wastewater treated
through the secondary stage.
The solids in these sludges (primary and secondary) consist of contaminants that
were contained in the raw sewage together with living and dead microorganisms pro-
duced during secondary treatment. Coming from the clarifiers, the sludges contain
pathogens, toxic chemicals and metals, putrescible organic, plant nutrients, and a lot
of water. It is necessary to treat sludge before releasing it into the environment.
Further concentration or dewatering makes treatment and ultimately disposal
Many processes are used in treating sludge to meet disposal requirements, plant
management and cost constraints, or other criteria. Two of the basic functions of sludge
treatment are reduction of water content and digestion of organic to reduce odors and
As liquid is separated, it is usually combined with the wastewater entering secondary
treatment. The remaining solids (about 1 to 1.5 tons dry weight, per one million gallons
treated) may be incinerated, placed in a landfill, or utilized as fertilizer.
Characteristics of Secondary Effluent and Sludge
The treatment processes described thus far are usually the minimum sewage treat-
ment required in Florida. Secondary effluent and processed sludge can be released to
the environment'under controlled conditions without endangering public health. Both
secondary effluent and sludge contain a long list of substances that tend to vary greatly
in quantity. Some are nutrients of plants and/or animals, but are toxic when in large
quantities. Others are toxic even at low concentrations. While most references do not
associate hazardous levels of toxic materials (excluding pathogens) with effluent,
sludges may contain significant quantities. The level of toxic metals, such as cadmium,/
limits the amount of sludge that can be landspread at a given site. The regulation
governing landspreading of sludge is Rule 17-7, Florida Administrative Code (FAC).
Fortunately, effluent and sludge from predominantly residential sewage contain fair-
ly predictable amounts of mostly non-toxic materials. Table 1 presents the major plant
nutrients in "typical" effluent and sludge. It is important to recognize that a particular
sewage treatment plant may produce effluent and sludge nutrients quite different from
the values in Table 1. Plants that are treating sewage flows below their design capacity
will usually produce effluent with lower nutrient content.
In some coastal cities, sewage collection lines run through salty groundwater. Leaks
at pipe joints and manhole connections sometimes allow salt water to enter the system
and cause unusually high salt content in the treated effluent. Some ornamental plants
can be damaged if irrigated with salty effluent.
Table 1. Major Plant Nutrients Contained
in Secondary Effluent and Sludge1
NUTRIENT EFFLUENT SLUDGE
(ppm) (percent of dry weight)
Nitrogen, as N 25 4.0
Phosphorus, as P 10 1.1
Potassium, as K 82 0.8
1 Adapted from Metcalf and Eddy, 1979, Wastewater Engineering.
SEstimated from several sources.
Advanced Wastewater Treatment
The preceding discussion of the characteristics of secondary effluent and sludge
points out the materials remaining in wastewater following secondary treatment.
Discharge of this effluent to some waterways can cause over-enrichment by the plant
nutrients in the effluent, thus necessitating further treatment to reduce one or more of
the major nutrients. Treatment beyond secondary is sometimes called tertiary treat-
ment, but since one or several additional processes may be involved, the more common
term is advanced wastewater treatment (AWT). Landspreading can be considered a form
of AWT that is often used as an alternative to other more expensive and energy inten-
sive processes for removing plant nutrients.
Nutrients are in wastewater as dissolved and suspended solids, and nitrogen may be
in the form of dissolved ammonia. Several methods have been developed to remove one
or more nutrient compounds. They include ammonia stripping, nitrification, chemical
precipitation, and filtration.
Filtration removes suspended solids from secondary effluent using granular beds or
screens. In addition to clarifying the final effluent and reducing BOD and nutrient con-
tent, filtration increases the effectiveness of disinfection, resulting in a virtually
\ pathogen-free effluent. This method is being used in Florida and is greatly enhancing
the usefulness of effluent for golf course and urban irrigation.
Regulations for Landspreading
The collection, treatment, and disposal of domestic sewage is regulated by the Florida
Department of Environmental Regulation (DER) under Chapter 17-6 of the Florida
Administrative Code (FAC) entitled Wastewater Facilities. Requirements for the
design, operation, and maintenance of effluent land applications are contained in a
companion manual entitled Land Application of Domestic Wastewater Effluent in
Florida, which is incorporated in the wastewater facilities rule as Section
A landspreading system to utilize effluent for irrigation would be classified as a "slow
rate" system by the DER. Other land disposal systems include: rapid rate, which
involves effluent discharge to level, diked areas of permeable soils for rapid infiltra-
tion; overland flow, which utilizes sloping vegetated areas for nutrient uptake; absorp-
tion fields, which operate like septic tank drainfields; and percolation-evaporation
ponds, which are common with small treatment plants. Percolation-evaporation ponds
often function as flow-through ponds due to soil sealing and poor evaporation. They are
not usually considered a landspreading method.
Because percolation systems do not provide for a "harvest" of nutrients, their use
near sensitive waters and aquifers would be restricted unless advanced levels of treat-
ment are performed prior to land application. Slow rate systems, on the other hand, are
designed for some water and nutrient removal by a growing crop. Uptake rates for
Nutrients, as well as water, become important for slow rate landspreading.
The level of wastewater treatment required prior to landspreading with a "slow rate"
system depends upon the extent of public access, the planned level of operational
management, the location relative to ground and surface waters, and other factors.
Chapter 403, Florida Statutes, requires a minimum of secondary treatment for all
domestic wastes. "Secondary" effluent requirements are variably defined in Chapter
17-6, FAC, with limitations ranging with the type of system to be used.
In addition to secondary treatment, basic disinfection is required for most systems.
This allows no more than 200 fecal coliforms per 100 milliliters of effluent on an annual
basis. These minimum preapplication treatment levels are acceptable for slow rate
landspreading with restricted public access and crops not intended for human con-
Preapplication treatment beyond the minimum is required where unrestricted
S public access is involved, such as for most park and golf course applications. The
effluent must contain not more than 5 milligrams/1 milligram of suspended solids, and
no detectable fecal coliforms. This requirement is intended to minimize public health
concerns over pathogens, including viruses. The filtration process following secondary
treatment (activated sludge or extended aeration) and prior to disinfection can provide
this effluent quality.
Slow rate application systems must have facilities to provide storage for a minimum
of three days' effluent flow in order to accommodate rainy days or other times when
landspreading is delayed. Holding ponds must be sealed and designed with adequate
freeboard and emergency overflow facilities. With highly treated effluent, ornamental
ponds in parks and on golf courses might be used for this requirement.
Buffer zones are required between the effluent application site and surface waters or
drinking water wells. Depending on treatment levels, buffer zones of 100 to 500 feet in
width may be required.
Slow rate spray-irrigation systems must have a minimum 100-foot buffer between
the application area and public outdoor eating, drinking and bathing facilities,
/ regardless of the level of preapplication treatment. This requirement is an important
design consideration for parks and golf courses with picnic tables and drinking foun-
Monitoring wells are necessary for slow-rate landspreading systems with average
daily flows exceeding 100,000 gallons. This is sufficient to irrigate only 13 acres at 2
inches per week, so most golf course systems would require monitoring. At minimum,
i monitoring requires one background well outside of the area influenced by the systems
and a compliance well within the area of influence. Well design and sampling criteria
are provided in the land application manual.
Application for a permit for a landspreading system must be supported by an engineer-
ing report on the proposed site, including information on soils and hydrogeology. In addi-
tion, all proposed systems are required to maintain an operation and maintenance
manual and provide reports on the operation of the project.
Where effluent is to be landspread on lands not under the permitted's ownership or con-
trol, a binding agreement between the parties involved is required. Usually the agree-
ment extends for the useful life of the facilities. This feature is often a major hurdle for
placing slow rate effluent-irrigation systems on privately owned agricultural or urban
of Landspreading Effluent
In addition to understanding the quality of effluent produced by various methods of
sewage treatment and the regulations governing its use, it is important to understand
the relative costs and benefits associated with landspreading before deciding upon its
Table 2. Sewage Treatment Effluent Quality and Costs
Based on 10 mgd plant
Constituent' Raw Secondary Secondary Plus and Nitrogen
Sewage Treatment Filtration Removal plus
Cost2 Carbon Adsorption3
BOD mg/1 250 20 7 <1
COD mg/1 500 60 50 8
SS4 mg/1 300 25 7 <1
N mg/1 35 25 20 2-10
P mg/1 12 10 8 .1-1.0
Kmg/1 10 8 7
Dollars per 1000 gallons
of sewage treated .74 .85 1.61
Dollars per month
per family -7.88 9.05 17.15
Constituent levels (except K) adapted from Meltcalf and Eddy, 1979, Wastewater Engineering.
2 Costs in 1981 dollars adapted from Culp-Wesner-Culp, 1979, Water Reuse and Supply, Courtesy D.W. York, DER.
3 Removal of N and P can be adjusted to meet discharge requirements; costs reflect stringent requirements.
4 Suspended solids.
Sewage Treatment Costs
Although costs will vary with plant size, plant location, and other variables, the
figures presented in Table 2 show how the cost of sewage treatment increases with the
level of treatment achieved. Increasing the level from secondary treatment to an
advanced wastewater treatment level, which reduces phosphorous and nitrogen for
environmental protection, raises the cost from $.74 per 1,000 gallons of sewage treated
to $1.61 per 1,000 gallons. For a family with an average daily flow of 350 gallons, this
represents an increase in sewage costs from $7.88 per month to $17.15 per month.
Given that increased treatment to remove nutrients substantially raises the cost of
sewage treatment, it is possible to understand why agencies responsible for sewage
disposal find the prospect of utilizing effluent for irrigation a potentially attractive
alternative. Assuming a plant used filtration for pathogen control, the agency would
then be able to spend up to $.76 per 1,000 gallons of treated wastewater on land-
spreading and would still be no worse off economically than with the highest level of
treatment. If landspreading costs anything less, they would be better off. Table 3
presents treatment cost differentials derived from Table 2 for different uses of effluent.
Table 3. Treatment Cost Differentials
10 mgd plant
Potential Cost Increase over Secondary
Treatment Use $/Thousand Gal. $/Mo./Family
Secondary with limited
(agriculture) 0 0
access .11 1.17
Nutrient Discharge to
Removal potable water
sensitive ecosystem .87 9.27
Value Of Sewage Effluent For Urban Irrigation
It is possible that the user of the effluent would be willing to pay something to receive
it because of its value for irrigation and as a fertilizer. The value of effluent used for
irrigation instead of another water source is shown under several different conditions
in Table 4. The value of nutrients in effluent is shown in Table 5.
Golf courses and other large green-space areas associated with urban development
are potential users of effluent. An example of the value involved can be seen by con-
sidering a golf course applying 50 inches of irrigation per year from a private well.
Table 4. Value of Effluent as an Urban Irrigation Source
Replace Public Replace
Supply' Private Well'
$.25 $.50 $.75 $1.00
Low Level 25 $ 170 $ 340 $ 510 $ 680 $ 48
High Level 75 510 1020 1530 2040 144
Low Level 50 350 680 1020 1360 96
High Level 100 700 1360 2040 2720 192
Parks, Green Space 35 240 480 720 960 67
' Based on reported typical use, which tends to be higher in South Florida.
SPublic supply cost, dollars per 1000 gallons.
3 Pumping cost at $.07 per thousand gallons.
From Table 4, the benefit for each acre irrigated would be $96 per year strictly due to
the substitution value of effluent for irrigation purposes. In addition to this, 50 acre
inches of "typical" effluent will also contain 220 pounds of nitrogen, 90 pounds of
phosphorous, and 80 pounds of potassium (Table 5). Assuming the golf course is capable
of utilizing all the nutrients provided, an additional $146 worth of benefits per year for
each acre irrigated will be realized by the golf course. In the example, this means that
effluent treated for pathogen control and delivered under pressure to replace private
well water would be worth up to $242 per year for each acre irrigated at 50 acre inches
per year. The unit value is $.07 (water value) plus $.11 (nutrient value), or $.18 per 1,000
gallons of effluent delivered to the golf course.
Table 5. Value of Effluent as a Nutrient Source
Nutrient Value Secondary' Secondary + Filtration2
$/# #/Acre-inch3 $/Acre-inch #/Acre-inch $/Acre-inch
N .30 5.66 1.70 4.43 1.33
P .70 2.26 1.58 1.81 1.27
K .20 1.81 .36 1.59 .32
($0.13/1000 Gal.) ($0.11/1000 Gal.)
' 25, 10,8 ppm N, P, K
2 20, 8, 7 ppm, N, P, K
3 1 Acre-inch = 27,150 Gallons
Another example of effluent value would be for irrigating home landscape. Assuming
water is purchased from a municipal system at $.75 per 1,000 gallons, and a typical
homeowner irrigates one-third acre at 25 inches per year, the value of effluent as a
water source would be $170 annually (one-third of $510, Table 4). Nutrients contained
in the effluent would be worth an additional $24 annually (Table 5, 8.3 acre inches at
$2.92). The total value to the homeowner is $194 per year, or $16 per month. The unit
value of the effluent in this example is $.75 (water value) plus $.11 (nutrient value), or
$.86 per 1,000 gallons delivered.
These examples show that the value of effluent is most dependent on the water source
it replaces. When it is substituted for potable water from a municipal system, its value
is significant. Another important aspect of value to the user is the fact that the use of
treated effluent represents a direct reduction in water withdrawal from the local sup-
ply. During water shortages, effluent can be used for irrigation while cutbacks may be
ordered if potable supplies are being used. Water stress and damage to valuable land-
scape plants may be averted with an assured supply of irrigation water.
Cost of Delivering Effluent
In order for effluent to be delivered to users, transmission lines will have to be con-
structed to large users and, in the case of homeowner use, an additional water distribu-
tion system will be needed within the residential area. These costs must be determined
for each project and will vary according to the distance the effluent has to be
transported and the volume of effluent being used. Some typical costs for effluent
transmission and distribution are shown in Table 6.
Table 6. Cost of Effluent Delivery
Transmission Distance Effluent Pumped Total Cost
(miles to users) (mgd) ($/1,000 gal)
5 1 .41
10 10 .23
15 50 .18
within subdivision $62/yr., ea. lot*
Based on distribution system at $500/lot, 30 yrs. at 12%
Summary of Benefits and Costs
For the golf course and home landscape irrigation examples, the benefits and costs
may be summarized as follows (Table 7), assuming the effluent source to be a 10 mgd
plant, 10 miles distant.
In these examples, both parties benefit the users receive benefits from the irriga-
tion and fertilizer value of water, and the sewage treatment agency receives benefits
from reduced costs. In addition, society as a whole has the potential of receiving
benefits from the fact that water has been conserved. If the effluent users were charged
for the water, they might be expected to pay up to the equivalent value of irrigation and
One major drawback to using irrigation for effluent disposal is the fact that sewage
flow does not stop during rainy periods when irrigation is unnecessary and possibly
even detrimental. If unneeded effluent is forced upon the user, the benefits received by
the user would have to be reduced according to the inconvenience and losses incurred.
The problem of temporary excess effluent can be solved by providing alternative short-
term disposal, storage for the effluent, or extension of the irrigated area for very low
application rates. All these alternatives represent costs that would have to be included
in the evaluation of economic feasibility oflandspreading as an alternative to advanced
Table 7. Benefits and Costs
Golf Course Use of Effluent
Reduced treatment $.76
Transmission to golf course $.23
Value for irrigation .18
Totals $.94 $.23
Net Benefit $.71 per 1000 gallons
Home Landscape Use of Effluent
Reduced treatment $ .76
Transmission to Subdivision $.23
Distribution in Subdivision .26
Value for Irrigation .86
Totals $1.62 $.49
Net Benefit $1.13 per 1000 gallons
Summary and Conclusions
Landspreading sewage effluent is often an economical alternative to treating
wastewater to remove most nutrients. Rapid rate infiltration basins, when per-
missable, are less costly than slow rate, irrigation-type systems. Utilization of effluent
for irrigation and fertilization of urban landscapes offers several benefits not
associated with rapid infiltration. Tables 4 and 5 show water nutrient values that are
significant for urban areas, particularly where expansion will allow economical
installation of effluent distribution systems concurrent with the potable water
systems. In addition, re-using wastewater is increasingly important for water conser-
Although it is difficult to assign an economic value to irrigation assurance during
water storage of unknown frequency, the value could be significant in terms of protect-
ing valuable plantings and maintaining customer satisfaction. As coastal water sup-
/ plies become more critical, pressure from water management officials and others adds
Sto the attractiveness of re-using wastewater for the irrigation of urban landscapes
without depleting municipal water sources.
This publication was promulgated at a cost of $1,755.05 or 12
cents per copy, to inform Floridians about an alternate water'
source in urban areas. 10-15M-86
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTIlTjTE
OF FOOD AND AGRICULTURAL SCIENCES, K.R. Tefertiller, director, ih' b'pera-
tion with the United States Department of Agriculture, publishes this information
to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and is
authorized to provide research, educational information and other services only ...........""" .
to individuals and institutions that function without regard to race, color, sex or national origin. Single
copies of Extension publications (excluding 4-H and Youth publications) are available free to Florida
residents from County Extension Offices. Information on bulk rates or copies for out-of-state purchasers
is available from C.M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida,
Gainesville, Florida 32611. Before publicizing this publication, editors should contact this address to