13. Green, O., Eyer, F., and Pierce, D., "Studies on Removal of Phosphates and Related Removal of Suspended Matter and Biochemical Oxygen Demand at Grayling, Michigan, March September 1967", Division of Engineering, Michigan Department of Health, Lansing, Michigan, and The Dow Chemical Company, Midland, Michigan (1967).

14.

"Studies on Removal of Phosphates and Related Removal of Suspended Matter and Biochemical Oxygen Demand at Lake Odessa, Michigan, May October, 1967", Wastewater Section, Division of Engineering, Michigan Department of Public Health, Lansing, Michigan, and The Dow Chemical Company, Midland, Michigan (1967).

15. Dow Chemical Company, Private Communications (1968).

16. Black & Veatch, Unpublished Data ((1970).

17. Condon, W. R., “Design of the Wyoming, Michigan Wastewater Treatment Plant Improvements", Presented at the Technical Seminar/Workshop on Advanced Waste Treatment, Chapel Hill, North Carolina (Feb. 9-10, 1971).

18. "Report on Wastewater Treatment Plant Improvements, Wyoming, Michigan", Black & Veatch of Michigan, Consulting Engineers, Kansas City, Missouri (1970).

19. Stonebrook, W. J., Dykhuizen, V., Beeghly, J. H., and Pawlak, T. J., "Phosphorus Removal at a Trickling Filter Plant Wyoming, Michigan", Unpublished (Undated).

20. Johnson, E. L., Beeghly, J. H., and Wukasch, R. F., "Phosphorus Removal with Iron and Polyelectrolytes", Public Works, 100:11, p 66 (1969).

21. Cherry, A. L., and Schuessler, R. G., "Private Company Improves Municipal Waste Facility", Wat. and Wastes Eng., 8:3, p 32 (1971).

22. Boggia, C., and Herriman, G. L., "Pilot Plant Operation at Warren, Michigan", Proceedings of the 43rd Annual Conference of the Michigan Pollution Control Association (1968).

23. Gaughan, D. M. and Alvord, E. T., "Phosphorus Removal by Ferrous Iron and Lime", Final Report by Rand Development Corp. for the County of Lake, Ohio and EPA's Water Quality Office on Federal Grant No. 172-01-68, Prepublication Copy (1971).

24. Progress Reports, Mentor, Lake County, Ohio, FWPCA Grant WRPD 172-01-68, May 15, 1968 through May 31, 1969 and September 1, 1969 through July 31, 1970.

25. Voshel, D., and Sak, J. G., “Effect of Primary Effluent Suspended Solids and BOD on Activated Sludge Production", JWPCF, 40:5, Part 2, p R203 (1968).

Chapter 5

PHOSPHORUS

REMOVAL BY LIME ADDITION
BEFORE THE PRIMARY SETTLER

5.1 Description of Process

Addition of lime to raw wastewater for the purpose of phosphorus precipitation can be applied to conventional plants using the activated sludge process. Because the pH of the primary effluent ahead of the activated sludge process should not exceed a value of 9.5 to 10.0, phosphorus removal by this method is generally limited to efficiencies of about 80%. Higher pH values can result in biological upset. Greater phosphorus removal, if necessary, may be accomplished by aluminum or iron addition to the aerator or final settling basin. One fortunate aspect of the lime clarification-activated sludge process is that the high pH of the primary effluent is reduced in the aeration tank by the CO2 produced by biological metabolism.

The removal of phosphorus in primary treatment is accompanied by increased BOD and suspended solids removal and thereby reduces the organic load on the secondary treatment facilities. Reduced organic load benefits overloaded plants and also aids in establishing nitrification. Some phosphorus removal is accomplished by biological treatment and this method takes advantage of that characteristic to remove some of the remaining phosphorus that would be more difficult to remove by chemical treatment. Use of lime rather than an iron or aluminum salt does not add large amounts of other ions such as sulfate or chloride to the treated water. Other possible benefits include improved oil, grease, and scum removal and less corrosion in the primary sludge handling system.

One consideration in selecting the lime clarification-activated sludge process is the ability to use existing primary settling basins for phosphorus removal. The initial capital expenditure for phosphorus removal, therefore, may be comparatively small There is a need for feeding lime, mixing of the lime with the water, and flocculation. Lime feeding equipment is discussed in Chapter 10. Equipment requirements for mixing and flocculation will vary depending on the configuration of the existing plant. In some cases, mixers and flocculators may have to be provided. In others, satisfactory flocculation will occur with no additional equipment.

Sludge disposal may become more complex and lime sludge may not be compatible with existing disposal processes such as anaerobic digestion. Furthermore, recovery of lime by recalcining which is discussed in Chapter 8 may not be practical with this process because of the low CaCO3 content of the sludge.

5.2 Process Performance

The concept of using lime for phosphorus removal in primary treatment preceding activated sludge was advanced a number of years ago. Since then it has been investigated at laboratory scale and full plant scale.

Albertson and Sherwood (1) established with laboratory data that lime treatment of raw wastewater is an efficient means of removing 80% or more of the phosphorus and 60 to 70% of the BOD. Discharge of the pH 9.5 or above water to a complete-mix activated sludge system eliminated the need for pH correction because of CO2 production by the biological activity in the aerator.

Schmid and McKinney (2) investigated lime precipitation of phosphate from wastewater on a laboratory scale. Their studies also included precipitation of various phosphate compounds from distilled water solutions by the addition of lime and consideration of the dewatering characteristics of the sludge formed by phosphate precipitation. These investigators concluded that both orthophosphate and polyphosphates, normally present in wastewater, can be precipitated readily with lime. They also noted that the complex phosphates present in wastewater seriously inhibit calcium carbonate precipitation. In view of this, they concluded that sludge recalcination for lime recovery would not be practicable. For the wastewater employed in the test work, the addition of 150 mg/1 of lime (calcium hydroxide) resulted generally in a pH of 9.5, total phosphorus removal before biological treatment of 60%, and suspended solids removal of 90%. The authors concluded that sludge produced by primary treatment, where lime is employed, may be about twice that obtained normally. Overall sludge production, including waste activated, would be about 1.5 times normal. The process can be controlled by operating at a constant pH. Operation of a complete-mix activated sludge system is not hindered, provided proper control is exercised. Microbial CO2 production in the activated sludge unit is sufficient to maintain the pH near neutral. A mixture of lime-precipitated primary sludge and waste activated sludge will dewater and filter well with small additions of anionic polymers.

Following extensive laboratory jar test and bench-scale studies, Black and Lewandowski (3) conducted a plant-scale investigation of lime treatment for phosphorus removal at the 180,000-gal. (imperial) per day Richmond Hill, Ontario activated sludge plant. Lime was added before the primary clarifier on a continuous basis for approximately 3 months. In this case, no special mixing or flocculating equipment was provided. The raw wastewater had an alkalinity of 381 mg/1 as CaCO3, a total hardness of 401 mg/1 as CaCO3, and an average total phosphorus content of 10.6 mg/1 as P. Normal plant operation produced a total phosphorus removal of 39% with BOD and suspended solids removals in the primary unit of 21% and 37%, respectively. A lime dosage of 175 mg/1 was sufficient to produce a pH of 9.3. During lime addition, BOD and suspended solids removals in the primary clarifier were increased to 72 and 78%, respectively, and total phosphorus removal averaged 83%. Phosphorus removal over the total plant was 92% resulting in a total phosphorus concentration in the secondary effluent of 0.9 mg/1. Throughout the study, CaCO3 accumulated on the effluent weirs and outlet troughs of the primary clarifier but was easily removed with a water hose. There was no appreciable buildup on submerged structures within the primary unit. Lime treatment had no detrimental effects on the secondary process. The high pH of the primary effluent was promptly reduced by the CO2 generated in the aeration tank. Sludge equilibrium was reached and maintained throughout the study. As a result of improved primary performance, the organic loading to the secondary plant was reduced,

resulting in its behavior as an extended aeration process with no secondary sludge being wasted. If sludge wasting had been required this would have increased the phosphorus removal.

cium

The sludge precipitated in the primary clarifier was a mixture of a calphosphate compound, Ca(OH)2, CaCO3, and coagulated organics plus grit. This sludge was free flowing and considerably different from the gelatinous type sludge produced in bench-scale studies. The sludge concentrated readily to 10 to 12% solids by gravity thickening but was very difficult to dewater. The sludge removed from the primary unit had a solids content of 6 to 8%. The investigators concluded that there would be no practical value in attempting to digest the sludge derived from the lime treatment process, inasmuch as biological activity of the sludge is suspended by the high lime content. The sludge is free from noxious odors even after prolonged holding under anaerobic conditions. The authors suggest that the sludge might be dewatered by polymer conditioning and vacuum filtration or centrifugation followed by landfill disposal.

The study results indicated that the organic capacity of an existing plant could be increased without jeopardizing good treatment through the addition of lime to the primary units. Laboratory studies using a combined flocculation-sedimentation system with high rates of primary sludge return showed that recirculation of the lime sludge increased the efficiency of the lime and improved effluent clarification. Supernatant from the chemical sludge, even after 5 weeks of storage, contained no appreciable amount of phosphorus, either total or soluble.

It was concluded that the lime treatment process could be readily controlled in response to flow or on the basis of pH in a flash mixer or in the primary effluent. It was also concluded that lime control in this manner would be simpler than that of a chemical dependent on incoming phosphorus concentration.

During the period of January to April 1967, treatability studies for phosphorus removal were made on the influent to the Rochester, New York treatment plant (4). At this time the raw wastewater phosphorus concentration averaged about 8 mg/l as P. Studies showed that the activated sludge process removed about 20% of the influent phosphorus. This removal agreed reasonably well with the phosphorus uptake normally experienced with biological metabolism.

Jar tests were performed to determine the chemical treatment necessary to achieve the State requirements of 80% phosphorus removal. Lime, and ferric chloride with anionic polymers were used in the jar tests. Either 100 mg/1 of lime as CaO (pH 9.5) or the combination of 40 mg/1 FeCl3 and 0.5 mg/1 Dow A23 was found to reduce the phosphorus concentration of the supernatant by about 70%. Adding the removal from the biological treatment would give an overall phosphorus removal of at least 80%. BOD and suspended solids reductions indicated by these jar tests were 50% and 80 to 90%, respectively. Lime was selected as the chemical to be used.

The present treatment plant facilities consist of settling tanks and Imhoff tanks, sludge holding tanks, coil filters, and sludge incinerators.

Expansion plans call for modification of present settling basins with flocculation sections, addition of settling basins incorporating flocculation sections, conversion of the Imhoff tanks to complete-mix activated sludge units, chlorination, and discharge to Lake Ontario through a new 10 ft diameter 18,000 ft outfall line. Detention time in the flocculation sections of existing settlers will be 20 minutes. In the new basins, the flocculation time will be 40 minutes. Existing and new settlers will have overflow rates 2 of 1,000 gpd/ft2 and detention times of 2.3 and 2 hours, respectively, at the average design flow. A schematic diagram of the expanded plant is shown on Figure 5-1. Plant design is based on the following criteria:

300

240

Suspended solids concentration, mg/1

Volatile suspended solids concentration, mg/1

Lime will be added just after the waste passes through the comminutors. The pH will be monitored ahead of the flocculation basins and lime addition will be automatically regulated to maintain a pH of 9.5.

Anionic polymer will be fed at a point just ahead of the flocculation basins when necessary to improve settling. Provisions for returning solids from the primary basins to the flocculation basins have been included to enhance the flocculation process. Although lime treatment together with biological treatment should remove 80% of the incoming phosphorus as required by the State, chemical facilities for addition of alum to the aeration tanks will also be provided. This system of phosphorus removal may be used alone when the lime system is not in operation or may be used to supplement lime clarification for greater phosphorus removal.

Since 80 to 90% suspended solids removal is expected in the primary basins, the quantity of primary sludge based only on the increased suspended solids removal will be about 1-1/2 to 2 times the normal sludge quantities. The sludge produced as a result of chemical precipitation must be added to this quantity. It is anticipated that, including the chemical sludge, about three times the normal amount of sludge will be produced with chemical treatment. The primary sludge quantities at design flow with lime treatment are estimated to be 175 tons/day. Without lime treatment the primary sludge quantities will be about 65 tons/day. Since the load on the activated sludge portion of the process will be reduced by about 50%, the quantity of waste activated sludge, which is very difficult to dewater, will also be reduced proportionally. At design loads with lime treatment, the waste activated sludge quantities are estimated to be about 30 tons/day as compared to about 75 tons/day if lime treatment were not used. The solids handling facilities have been designed for a capacity of 270 tons/day.