Aerial view of the Warren, Maine lagoon system. Photo courtesy of Woodard and Curran.

Lagoon Systems In Maine 

Systems In Maine

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Mars Hill Wastewater Lagoon System - Mars Hill  Maine. Photo Courtesy of Wright-Pierce Engineers.
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Municipal Wastewater Lagoon 
Phosphorus Removal


Phosphorus (P) occurs in natural waters and in wastewaters almost solely as phosphates. These phosphates include organic phosphate, polyphosphate (particulate P) and orthophosphate (inorganic P). Orthophosphates are readily utilized by aquatic organisms. Some organisms may store excess phosphorus in the form of polyphosphates for future use. At the same time, some phosphate is continually lost in the sediments where it is locked up in insoluble precipitates.

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.


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).


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).


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.


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.


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.


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.


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



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