FEATURE
I
Stacy L. Daniels and Daniel G. Parker The Dow Chemical Co., Midland, Mich. 48640
Many species of aquatic photosynthetic organisms utilize water-soluble anions of phosphorus as nutrient sources. Rampant growths of these organisms produce undesirable tastes and odors in potable waters, contaminate recreational areas. and restrict populations of more desirable organisms such as fish and man. Some legislatures have or are considering bans on the sale of detergents containing phosphorus builders to limit the input of phosphorus to receiving waters. Phosphorus removal must still be practiced in the treatment of municipal waste waters, however, since other sources of phosphorus cannot be controlled at their sources. Significant reductions of all phosphorus species regardless of source are technically feasible in existing waste water treatment plants. These reductions can be accomplished by modifying conventional primary or secondary treatment to include chemical precipitation and flocculation processes. Phosphorus removal by chemical means is currently practiced in the Great Lakes area on a wide scale and is seriously being considered in other areas.
Sources and distribution of phosphorus species Three major classifications of phosphorus present in untreated municipal waste water are shown in Figure 1. Suspended phosphorus originates from human waste, food scraps, and insoluble inorganic materials. All animal and plant cells contain 3 3 % phosphorus in both organic and inorganic forms. Polyphosphorus consists mainly of pyro- and tripolyphosphates which are the familiar builders used in detergent formulations. This species is present in waste water in the dissolved form. Orthophosphorus, also present in the dissolved form in waste water, is derived directly from simple inorganic salts or indirectly as a degradation product of organic phosphorus compounds or soluble condensed phosphates. Poly- and suspended phosphorus can also be degraded by biological means or chemical hydrolysis into the ortho form. The relative concentrations of these three species of phosphorus can vary with the age and source of the waste water and the degree of treatment. The use of detergents has been variously estimated to contribute 3070% of the total phosphorus present in an average waste water. To control eutrophication, a major portion of all species of phosphorus present in the waste water should be prevented from reaching lakes and rivers. Phosphorus removal, therefore, must be practiced at waste water treatment plants even if there is no detergent phosphorus contribution to the waste water. Chemical precipitation/flocculation Chemical processes for removing dissolved and suspended phosphorus species are effective in removing all forms of phosphorus at conventional waste water treatment plants. Biological processes for phosphorus removal are generally limited in their applicability. Chemical 690
Environmental Science & Technology
precipitation and flocculation are usually applied in two process steps. The first step consists of adding inorganic coagulants such as ferric or ferrous iron, aluminum, or calcium salts to the waste water. Typical inorganic coagulants used for this purpose include iron chloride, iron sulfate, aluminum sulfate, sodium aluminate, and calcium oxide. The cations from these salts combine with the dissolved phosphate anions present in the waste water to form insoluble metal-phosphate precipitates which remain in suspension. The metal-phosphate precipitates are small, colloidally stable, and usually cannot be separated entirely from the waste water by conventional sedimentation. The second process step, therefore, consists of adding small amounts of an anionic, water-soluble polyelectrolyte floc-
FIGURE 1.
Phosphorus distribution in municipal waste water
Untreated municipal waste water contains many forms of phosphorus, all of which contribute to eutrophication of U.S. lakes and rivers; however, chemical processes, such as precipitation/ flocculation, are effective in removing all forms of phosphorus at conventional waste water treatment plants
culant which collects and binds together the small phosphate precipitates into large, readily settleable flocs. These flocs settle rapidly and effectively in conventional primary sedimentation tanks. Split addition of the precipitating/flocculating chemicals-chemical addition at two or more locations within a given plant, such as before primary and before secondary clarification-will obtain greater removals.
Chemical feeding systems Systems for feeding chemicals to obtain phosphorus removal are not complex and can easily be designed and installed. Inorganic coagulants are available as dry powders or granules, or as liquids containing 25-50% dissolved solids. Bulk transport of solutions is usually the most economical means of shipment and the most convenient form to handle. Solutions of coagulants are not as viscous as flocculants and can easily be prepared and metered into the waste water flow. Specific construction materials designed to resist corrosion are required to contain these acidic metal salt solutions; therefore, common metals are not recommended. Exposed surfaces of storage tanks, pipes, and pumps should be made of glass fiber-reinforced plastic, rubber, or polyvinyl chloride. A manually operated disperser often is used for preparing solutions of dry flocculant when treating relatively small volumes of waste water. This consists of a funnel or suction hose connected to a water aspirator which draws in dry flocculant and delivers a partially wetted suspension to a holding tank. More sophisticated feed systems are available for larger installations including an automatic flocculant disperser. A variety of automatic dispersers are available which can be programmed to prepare solutions at desired rates and concentrations. Solutions of polyelectrolyte flocculants are highly viscous, and ordinary flow-regulating valves and meters are usually not adequate to control the feeding of small volumes of solutions. Since polyelectrolyte solutions exhibit
non-Newtonian flow, special considerations should be made so that pumps and lines are not oversized on the basis of Newtonian flow properties. Solutions of flocculants are noncorrosive to most common metals. Most standard materials of construction, such as polyvinyl chloride or common metals, can be used for all equipment; galvanized materials should be avoided.
Safety and environmental aspects Polyelectrolyte flocculants are relatively easy to handle. Slippery conditions can arise, however, if spills of flocculant solutions or dry powder occur. Also, accumulated dust can eventually present a hazard. Minor spills should be flushed immediately with water, and major spills may require application of neutralizing or absorbing agents. Containment dikes are recommended to surround storage tanks containing inorganic coagulants. Inorganic coagulants are only slightly irritating to intact skin if washed off promptly after contact. Organic flocculants are only slightly irritating to abraded skin but should also be washed off after contact to prevent accidents caused by slippery conditions. Safe handling of inorganic coagulants requires operators to wear goggles and rubber gloves during transfer; installation of eye and safety showers is recommended. Inorganic coagulants and organic flocculants, generally applied under controlled conditions, become associated with sludges formed during water and waste water treatment, Inorganic coagulants are not toxic to aquatic organisms unless extreme pH changes are produced when excessive quantities are used. Organic flocculants have relatively low oral toxicities, and many have been declared suitable for potable water treatment by the U.S. Environmental Protection Agency. Anionic and nonionic flocculants generally are not toxic to fish. Cationic flocculants can be toxic to fish if used incorrectly in low solids systems but have been applied successfully in many applications following carefully Volume 7, Number 8,August 1973
691
TABLE 1
I nst r u mentation and cont roI s y ste m s for coaguI ation / f locc ul ation p r oces ses Average plant flow, 1 mgd = lo6 gallday
Coagulant addition system
Flocculant preparation system
Flocculant addition system
Manual operation; variable speed device with pump calibration and/ or rotameter Automatic operation; variable ratio station: feed proportional to sewage flow Same as immediately above with feedback correction and flow totalization
IO0
controlled conditions. Detailed information on the proper and safe handling of coagulant flocculant chemicals is available from the manufacturers. Instrumentation and control In spite of the heterogeneous nature of most waste water suspensions. continuous iquantiative monitoring and control of coagulation/flocculation processes has been achieved. instrumentation in both processes has been limited to controlling the storage and preparation of chemicals. the metering of chemical additions. and the monitoring of influent and effluent qualities. Process analyzers have measured turbidity. surface charges, or orthophosphorus concentration. There is no universally applicable instrumentation or cont ro I system for coag u I at i on / f I o c c u Ia t ion processes. Comparisons of typical flocculant preparation systems, flocculant addition systems. and coagulant adtlition systems for five ranges of total plant flow are sumtnarized in Table 1 . The overall accuracy of the instrumentation is increased as warranted with increased plant size. In a typical automatic control loop. the easily measured orthophosphorus concentration is first determined by an automatic analyzer. A sigriai proportional to the total phosphorus concentration (based on correlation of ortho- to total phosphorus) is developed by an electronic converter and multiplied by a flow rate signal to provide a third signal proportional to the total phosphorus load. The inorganic coagulant is then added proportions.lly to the total phosphorus load. The polyelectrolyte floisculant is simply added proportionally to flow. Operating and capital costs The cost of removing phosphorus is mainly for operation rather than for capital expenditure. Chemicals account for about t w o thirds of the total yearly operating cost. Differences in the chemical cost of phosphorus removal are caused by variations in the average total phosphorus concentration. the ratio of municipal /industrial 692
Environnental Science & Technology
waste, plant location, and type of inorganic salt selected. Ferrous and ferric chlorides have been most effective on a cost/performance basis in the Great Lakes area. Operating costs for chemical coagulation/flocculation processes installed in plants of different capacities are shown in Table 2. Chemical costs encountered in specific cities have been as low as 1.75/1000 gal and as high as 6.05/1000 gal. The average chemical cost for plants incorporating the precipitation/flocculation process for phosphorus removal is about 2.55/100 gal. This total includes about 25/1000 gal for the commodity inorganic coagulant and about 0.55/1000 gal for the organic flocculant which is a specialty chemical available from several suppliers. These chemical costs are nof limited to the Great Lakes area and will be realistic in any area where phosphorus removal is required on a large scale assuming similar transportation costs. Because the chemistry ' i n volved in the precipitation reaction is not stoichiometric, the absence of detergent phosphorus from waste water would reduce the demand for the inorganic coagulant and the cost of phosphorus removal by only 25-30Y. Other cost factors associated with phosphorus removal are operating labor, maintenance, analytical work, sludge handling, freight, and capital costs. Increases in operating labor, maintenance, and analytical costs are about 0.35/1000 gal of sewage treated. Initiation of the chemical precipitation/flocculation process to remove phosphorus in a currently efficient secondary waste water treatment plant will increase sludge production by approximately 20-40% owing to the creation of the mefalphosphate precipitates and the more efficient removal of other solids initially present in the waste water. Allocation of total sludge disposal costs on this basis implies an increase in sludge handling and ultimate disposal costs due to phosphorus removal averaging less than 1.0@/1000 gal of waste water treated. Ultimate disposal of phosphorus-containing sludges is compatible with most conventional incineration or landfill practices designed to avoid recontamination of the environment. Significant alterations of these existilly w,uy"ca=== y w y l I initiation of phosphorus removal are not indicated. The entire operational cost for phosphorus removal is about 3.85/1000 gal including 2.5@/1000 gal for the
Settling pond. After coagulants and ilocculants are added to waste water, phosphorus-containing flocs Settle rapidly
TABLE 4
Criteria and enforcement deadlines for phosphorus removal state or proVlnce
Michqan Illinois Indiana Minnesota New York Ohio Pennsylvania Wisconsin
Ontario
Effluent criterion for total phOSphOruS
Effective deadline
80% removal
All
80% removal
12-31-72 1-1-73 1-1-74 1-1-73
< I mgjl.
