Phosphorus is essential to the growth of
organisms and can be the nutrient that limits the primary use of a body of
water. In the case where phosphate is a growth-limiting nutrient, the discharge
of raw or treated wastewater or industrial waste as well as non-point source
runoff to a body of water may result in the stimulation of growth of
photosynthetic aquatic macro-and micro-organisms in nuisance quantities. As a
result, there is a continuing effort to control the amount of P compounds that
enter surface waters in domestic and industrial discharges as well as non-point
source runoff. With respect to domestic wastewater, there are two means by which
P is removed: chemical precipitation and the use of various biological treatment
processes. In a lagoon treatment system, phosphorus is also removed by
assimilation into the biomass of algae cells. As the alkalinity increases during
daylight hours, the phosphate is precipitated and settles out of the wastewater.
Generally, the effluent P concentration is less than half of the influent
wastewater concentration.
Municipal lagoon wastewater treatment
facilities which remove phosphorus by way of chemical addition are the subject
of this special evaluation project (SEP). The purpose of this project is
three-fold: (1) Evaluate the operating experiences of the above referenced
wastewater treatment technology; (2) Examine the degree of success of this type
of treatment in removing phosphorus; and, (3) Identify operational problems. In
order to obtain basic data for this project, thirty-two municipalities in
Michigan and Minnesota as well as respective State personnel, and the Ontario
Ministry of Environment were contacted.
CHEMISTRY OF P-REMOVAL
The lagoon treatment systems listed in the
Appendix utilize the addition of chemicals to precipitate the P from the
wastewater. Chemicals typically used for P removal include metal salts such as
aluminum sulfate (alum), and ferric chloride. Ferrous chloride, lime, and
various polymers are also used.
ALUMINUM SULFATE: The form of aluminum used
for P removal is alum, a hydrated aluminum sulfate or Al2(SO4)3 o 14H2O. The
chemical equation for the reaction of alum with phosphate is as follows:
Al2(SO4)3 o 14H20 + 2PO43- - 2A1PO4 + 3SO42- +
14H20
The factors that affect the actual quantity of
alum required to attain a specific P concentration include alkalinity and final
pH of the wastewater, ionic constituents such as sulfate, fluoride, sodium,
etc., quantity and nature of suspended solids, microorganisms, and the intensity
of mixing and other physical conditions extant in the treatment facility. The
optimum pH for P removal using alum ranges from 5.5 to 6.5, but in typical
wastewaters, it ranges from 6.0 to 9.0.
Ferric Chloride: The chemical equation
associated with the reaction of ferric chloride with phosphate is:
FeCl3 + PO43- - FePO4 + 3Cl-
Ferric chloride is most effective in removing
P when the pH ranges from 4.5 to 5.0, with typical values of 7.0 to 9.0.
The chemistry of phosphorus removal from
wastewater is further discussed in the EPA publications, DESIGN MANUAL:
PHOSPHORUS REMOVAL (EPA 625/1-87/001, September 1987), and DESIGN MANUAL:
MUNICIPAL WASTE STABILIZATION PONDS (EPA 625/1-83-015, October 1983), and a
report on PHOSPHORUS REMOVAL UPDATE: NEW INFORMATION GATHERED DURING EXAM
REVISION PROCESS by Judith Gottlieb - WDNR, et. al. (August 1989).
CANADIAN EXPERIENCES
The International Joint Commission Report of
1969 resulted in the development and implementation of the Province of Ontario,
Canada policy requiring that the total P content in waste stabilization ponds
(lagoons) be reduced to below 1.0 mg/l. Batch chemical treatment of wastewater
prior to discharge in seasonal retention lagoons was explored as one method of
removing phosphorus. In the early 1970's, the Ontario Ministry of Environment
initiated a series of research projects on nutrient control in sewage lagoons.
The reports generated from these projects provided the baseline information upon
which most applications of this technology have been designed. Continuous and
seasonal discharge lagoons were researched. Three coagulants - ferric chloride,
aluminum sulfate and lime - were field tested at various dosages. Ontario
Province personnel provided the manpower to handle chemical addition for these
tests. The size of the lagoons was typically five acres and above.
The chemicals were applied to and mixed into
the wastewater of the secondary lagoon cell through the use of three 16 foot
aluminum or fiberglass boats equipped with a 100 or 150 gallon tank, chemical
feed pump, and outboard motors. The pump injects the chemicals into the propwash
located at the stern of the boat. In distributing the chemical throughout the
lagoon, a grid-work pattern of boat travel was used. Boat speeds were adjusted
to maximize the amount of turbulence produced. The floc, formed by the chemical
precipitants was given a minimum of 15 hours to settle out before lagoon
discharge began. The discharge period lasted from 1 to 15 days with the lagoon
discharge cell being drawn down from six feet to two feet or less.
The conclusions reached from these initial
studies and long term experiences were - (1) batch chemical treatment of
seasonal lagoons achieved total P effluent of less than 1.0 mg/l; (2) effluent
quality from batch treated lagoons was comparable to or better than that
achieved by conventional secondary treatment; (3) alum and ferric chloride
applications produced consistently high quality effluents while lime
applications were not as effective in removing P; (4) outboard motorboat method
of application achieved good dispersal of the chemical and adequate mixing with
lagoon wastewater; and (5) batch chemical treatment is feasible for existing
lagoon treatment systems which have adequate retention time for winter storage
and also is effective in removing algae from lagoon wastewater if the chemical
dosage is sufficient.
Based on these studies, the Province of
Ontario has designed and successfully operated over 20 full-scale municipal
lagoon treatment systems using alum to precipitate the phosphorus. These systems
discharge on a seasonal basis (spring and fall).
OBSERVATIONS
As stated above, operators of thirty-two
municipal wastewater lagoon treatment systems with P removal in Minnesota and
Michigan as well as respective State Water Pollution Control Agency personnel,
the Ontario Ministry of Environment, and Regional office staff were contacted in
order to ascertain specific basic operating data. This data was used to
determine the operating experiences as well as measure the success of these
treatment systems in meeting P effluent limitation. The following is a
discussion of State specific observations.
MINNESOTA
Design criteria has been established by the
State which serves as a guide for consulting engineers in designing multi-cell
lagoon treatment systems. For primary cells, one acre of water surface should be
provided for each 100-120 design population. In addition, this cell should not
exceed a BOD loading of 22 pounds/acre/day. The secondary cell(s) is utilized
for storage and final settling and is designed at a minimum of one-third the
volume of the entire lagoon system. The storage capacity of the treatment system
should be deter- mined by both the average surface area and maximum operating
depth of all the cells. Typically, the cells should have sufficient capacity to
store waste- water for a minimum detention period of 180 days (covers the winter
season and sufficient time for winter to summer transition). The cells of the
lagoon treatment systems should be lined to retain the wastewater and to prevent
its intrusion into groundwater. Normally, clay liners are a minimum of one foot
thick. Other types of liners include vinyl and incorporated bentonite.
The State has eleven facultative lagoon
wastewater treatment systems which utilize the addition of liquid alum directly
into the secondary cells via motorboat in order to meet the total P effluent
limitation of 1.0 mg/l. These treatment systems have design flows ranging from
0.017 to 0.672 million gallons per day (mgd) with permitted seasonal (spring and
fall) discharge. The years of operation of these treatment systems ranges from 1
year to 7 years. The procedures used for addition of alum are very similar to
those utilized in Ontario, Canada. The alum is delivered in liquid form (tanker
truck) or dry form (bags) and is stored on-site. It should be noted that prior
to application, the dry alum is mixed with water to form a solution. The alum is
applied to the secondary cell by way of two methods. Both methods utilize a 12
to 17 foot boat equipped with a storage tank (55 to 500 gallon size), chemical
feed equipment, and an outboard motor ranging in size from 5 horsepower (hp) to
50 hp. In a majority of these cases, the alum is fed into propwash and is mixed
as a result of the action of the outboard (propeller driven) motor. However, in
two cases, the alum is sprayed onto the wastewater via outriggers on both sides
of the boat. The latter method, though ensuring full surface coverage, would
appear to not as thoroughly mix the alum with wastewater as would applying the
alum through the propwash, though adequate P removals are achieved.
The operators use two methods for determining
the appropriate alum dosage. One method involved the operator determining the
phosphorus concentration in the lagoon cells and matching the reading with those
in a precalculated chart. This chart lists the associated alum dosage which
would be applied to the lagoon wastewater at the level of phosphorus
concentration obtained in the sample. Then the dosage amount is applied in the
lagoon cell(s). The other known method involved the use of past experiences of
applying alum on the part of the operator. Should conditions change (e.g.
changes in phosphorus concentration), the operator will either add more or less
alum to ensure continued compliance with P effluent limitations.
The P-influent values for the eleven treatment
systems in Minnesota ranged from 1.5 mg/l to 6.0 mg/l with the average being
approximately 3.3 mg/l. The effluent levels for P for all these systems
regularly met the 1.0 mg/l effluent limitation. There were several minor
excursions (10 percent) above the limit but no pattern or specific cause was
discovered. The only exception was one facility with an infiltration problem
which required discharge in the middle of winter when the pond surface was
frozen.
MICHIGAN
Operators in Michigan have used a somewhat
different application of this technology at over 26 municipal lagoon treatment
systems currently in operation. The years in which these treatment systems have
been in operation ranges from 1 year to over 20 years. These include aerated as
well as facultative lagoons with the majority constructed with 3 to 6 lagoon
cells. These included not only systems designed for seasonal discharge (once or
twice a year), but also, continuous discharge systems (varying from 24
hours/day, 7 days/week to 8 hours/day, 5 days/week), as well as continuous
discharge lagoons where the chemicals are added to a clarifier following the
lagoon system. The sizes range from 0.25 to 7.5 mgd. None of these facilities
use motor boats to add the chemicals to the lagoons, but rather, they typically
rely on a mixing chamber located between the lagoon cells and clarifier.
Chemicals are added continuously or more specifically, whenever wastewater is
flowing through the mixing chamber. The P influent values for the 21 treatment
facilities in Michigan ranged from 0.5 mg/l to 15.0 mg/l with the average being
approximately 4.1 mg/l.
A wide variety of chemicals is used including
ferric chloride and alum. In addition, different polymers are used in
conjunction with the metal salts. A few treatment systems even have the
flexibility to add alum or ferric chloride alternately. The permit limitations
generally are written with a 1.0 mg/l effluent maximum based on a 30-day average
but several are based upon a pound per day maximum value or 30-day average pound
per day value.
The State does not have specific guidance for
designing wastewater treatment lagoons. However, as a minimum, the State advises
consulting engineers to use the criteria discussed in the Recommended States
Standards for Sewerage Works (i.e., Ten State Standards). In addition, the State
recommends that lagoon cells should be lined with either compacted clay or
synthetic liners with protective soil cover for protection of groundwater.
TESTS FOR TOTAL PHOSPHORUS
Generally, phosphorus analysis has two
procedural steps - the conversion of the phosphorus form of interest to
dissolved orthophosphate, and the colorimetric determination of dissolved
orthophosphate. Two types of analysis (tests) for total phosphorus are being
utilized in Minnesota and Michigan. Detailed information on both tests is given
in the fourteenth edition of the STANDARDIZED METHODS FOR THE EXAMINATION OF
WATER AND WASTEWATER (1976).
The ascorbic acid test, which is the
EPA-approved test for total phosphorus is most useful for routine wastewater
samples below 1.0 mg/l of phosphorus. The apparatus used for this test includes
colorimetric equipment, a spectrophotometer, a filter photometer, and
acid-washed glassware. Care needs to be taken when conducting this test due to
its sensitive nature in terms of time. The reagents are sulfuric acid, potassium
antimonyl tartrate solution, ammonium molybdate solution, ascorbic acid, and
standard phosphate solution. The principle behind this test is the reaction of
ammonium molybdate and potassium antimonyl tartrate with orthophosphate in acid
medium to form heteropoly acid hosphomolybdic acid. This acid is reduced to an
intensely colored molybdenum blue by the ascorbic acid.
Vanadate method or the vanadomolybdophosphoric
acid colorimetric method is useful for wastewater samples in the range of 1 to
10 mg/l of phosphorus. The
reagents are the standard phosphate solution,
hydrochloric acid or sulfuric acid, phenolphthalein indicator, potassium
sulfate, and the vanadate-molybdate reagent. The apparatus a spectrophotometer,
colorimetric equipment, a filter photometer, acid-washed glassware, filtration
apparatus, and filter paper. The general principle behind this test is the
formation of a heteropoly acid, molybdophosphoric acid resulting from the
reaction of ammonium molybdate in a dilute orthophosphate solution under acid
conditions. Yellow vanadomolybdo- phospheric acid is formed in the presence of
vanadium. The intensity of the yellow color is proportional to phosphate
concentration. The color remains stable for several days and its intensity is
unaffected by room temperature variations. This method is the easiest to conduct
for total P and is less sensitive than the ascorbic acid test.
CONCLUSIONS
The overall experience with these systems is
that the technology, in its various configurations, has been working very well.
Of the thirty-two lagoon treatment facilities reviewed as part of this report,
only two facilities are considered to be in significant noncompliance, though
one of these is not due to the technology but rather to excessive clearwater
entering the system resulting in discharges outside of the spring and fall
permitted discharge. The other facility in noncompliance has identified a
problem with resolebilization of precipitated phosphorus that the operator
believes is related to a change in pond pH caused by algal blooms. The chemical
equilibrium of precipitating phosphorus with metal salts is pH dependent but
none of the other facilities seemed to have experienced this phenomenon.
Most of the facilities though did report
typical lagoon operating problems. These included seasonal algae blooms which
are a common source of total suspended solids in the effluent, and mixing of
surface wastewater, algae, and duckweed which results in the resuspension of
precipitated solids as well as an increase in biological oxygen demand (BOD) and
suspended solids (SS). Other typical problems were associated with the handling,
storage and mixing of the chemicals which were discussed in the above referenced
EPA documents.
A number of minor instances have occurred at
some of the lagoon treatment systems where the actual effluent P value exceeded
the P effluent limitation (less than 10 per cent). Many of the Regional
treatment systems do a minimum of process control testing for adjustment of the
chemical dosage. Often, they rely on experience gained from discharges of past
years, adjusting the dosage in steps rather than by recalculating dosage based
upon phosphorus levels in the pond. Whether these minor permit violations are a
result of minimizing operator time at the facility, laboratory costs, and
chemical dosing or inadequate operation and maintenance could not be readily
determined. This varies somewhat from the Ontario experiences which depends upon
a larger dose of chemical to ensure permit compliance as well as reap a
secondary benefit of discharging less phosphorus (and BOD and SS that is also
precipitated) to the receiving waters.
None of these lagoon systems experienced
problems with buildup of sludges to levels which affected the effluent
concentrations. There were a few problems noted with localized sludge
accumulation within the lagoon. Accumulated amounts were an inch or less per
year, consistent with solids buildup in the primary lagoons cells. None of the
Ontario lagoons have had to remove sludge, but several did as part of lagoon
expansion projects.
Chemical addition on a continuous or batch
basis is easily calculated and applied through an influent structure or via
motorboat. The systems can and have regularly achieved effluent total phosphorus
limits of 1 mg/l or less under a wide variety of lagoon configurations, climatic
conditions, and a wide range of design flow rates (0.25 to 7.5 mgd). The
secondary benefits of chemical precipitation result in lower BOD and SS levels
in the effluent lagoon, which can partially counteract or overcome the
variability of algae and other suspended solids in the lagoon effluent resulting
in a more con-sistent permit compliance.
The addition of chemicals to an existing
lagoon via motorboat or mixing structure requires a relatively low capital
investment and operates quite well within many existing lagoon configurations.
It should be noted that as with any wastewater
treatment system, the type of treatment system discussed in this report depends
upon the operator's time, knowledge, and attention to ensure its proper
operation and maintenance.
This Report was prepared by
Charles Pycha and Ernesto Lopez
Environmental Engineers
Technical Support Section
Water Compliance Branch
U.S. EPA 5WCT-15-J
77 W. Jackson Blvd.
Chicago, Illinois 60604
(312) 353-2144
