phosphorus in chemical and physical treatment processes

Water Research Pergamon Press 1973. Vol. 7, pp. 145-158. Printed in Great Britain

PHOSPHORUS IN CHEMICAL AND PHYSICAL TREATMENT PROCESSES

K.-A. MELKERSSON

Boliden AB, Helsingborg, Sweden

Abstract--During recent years the activities for removal of phosphorus from sewage effluents have increased to a very high degree of intensity in those countries, where the phosphorus is one of the major factors contributing to man-made eutrification. The most successful approach to achieve this has been the erection and operation of plants for chemical treatment of the sewage. There already are many plants operating e.g. last year in Sweden ~ 110, Finland ~45, Switzerland ~40. The discussion will be limited to chemical and physical operations in plants of this kind.

Primary precipitation, secondary precipitation, simultaneous precipitation, pre-precipitation, post-precipitation and some techniques used for a single house or a small group of houses in rural and resort areas are considered. Particular design features, choice of chemicals, performance data, investment costs and operational costs are presented.

It is shown, that the removal of phosphorus, including the detergent phosphates, can be carried out efficiently and to very reasonable costs. A decrease in the phosphate content of the detergents does not give a proportional cost reduction in the treatment plant. Not only phosphorus but also BOD, suspended solids, bacteria, viruses, intestinal worm eggs and heavy metals are substantially reduced in the effluent water.

Some consideration is given to the sludge problem and the relationship between detergent composition and the performance of a treatment plant.

IN THIS paper the discussion will be limited to experiences obtained with chemical and physical processes of sewage treatment plants, in which phosphorus is removed by chemical precipitation using aluminium and iron salts and lime. The emphasis will be put on the experience and costs of full scale plants in operation mainly in Finland, Sweden and Switzerland and experiences from phosphorus control in rural and resort areas, when small units down to a single house are concerned.

GENERAL DEVELOPMENT

During recent years the increase in sewage treatment plants with phosphorus removal has been extraordinarily remarkable in those countries in Europe where the phosphorus is one of the major factors which contributes to troublesome man-made eutrophication. By the middle of 1971 there were in operation 110 such plants in Sweden, ~45 in Finland and ~40 in Switzerland. This trend will accelerate in the next few years. In FIG. 1 the expected development in Sweden is shown (SNV, 1972). By the beginning of 1973 some 230 sewage treatment plants equipped with phosphorus removal facilities will be in operation and by the beginning of 1975, 50 per cent of the population will be connected to sewage treatment plants with phosphorus removal and 80 per cent in densely populated areas, which means that the major areas where phosphorus removal is necessary, are covered.

Chemical precipitation of phosphorus in sewage treatment plants has started in several other European countries and in the U.S.A. and in Canada, particularly in Ontario, many plants will be in operation already in 1973.

In Sweden considerable progress has been made in obtaining solutions for treatment

145

146 K.-A. MELKERSSON

IOC"

7~

~. 5c

2~

I

nicol

ol Iteotrnenr

75 70/7[

Time

FIG. 1. Sewage treatment in Sweden 1960-1975. Percentage of population in urban areas.

of sewage from a single house or a group of houses in rural and resort areas, which also solve the phosphate problem, where they are of importance.

CHEMICALS USED

The chemicals of major importance in full scale plants are lime and salts containing aluminium or iron or both aluminium and iron. The most common aluminium and iron salts are aluminium sulphate, ferric chloride, ferrous sulphate and ferric sulphate--chloride. Ferric sulphate and sodium aluminate are also used in some cases and in Sweden, AVR, a granular product, particularly designed for sewage treatment in which aluminium sulphate is the main component, has obtained a dominating position.

JENKINS et al. (1971) have recently made a comprehensive survey of the chemistry and the performance of lime and aluminium and iron salts, when applied to sewage purification systems. In addition to this a few remarks ought to be added.

The performance of the chemicals cannot be sufficiently predicted from pure physico-chemical calculations in the complex systems of sewage effluents. The reason for this is that the compositions vary with time and from one place to another and many mechanisms are involved as chemical precipitation, flocculation and adsorption. This greatly influences the way in which the hydroxides and phosphates form and behave in practical plant operations. A compromise in the composition of a specific chemical or its application to a particular sewage situation has to be based on labora- tory tests on the particular- sewage or preferably on performance tests in pilot plant equipments.

Phosphorus in Chemical and Physical Treatment Processes 147

As shown by NILSSON (1969) the precipitation curves of orthophosphates with ferric and aluminium ions are not limited to the narrow pH-ranges obtained in pure water solutions, in sewage, where calcium ions are always present and interactions with sewage take place.

The presence of chelating agents in municipal sewage, which cannot be precipitated or are insufficiently degraded in the sewage treatment plant, can cause considerable carry-over of the ions of the precipitants, NTA and EDTA can carry undesirable amounts of iron ions to an iron sensitive receiving water as well considerably increase the transfer of certain heavy metals to the recipients (NILSSON, 1971).

When detergent phosphates are present they usually are mainly degraded to orthophosphates before they reach a large sewage treatment plant and are further degraded in the biological section of a treatment plant. The pyro-tripoly- and meta- phosphates are, furthermore, precipitated by the precipitants as shown by NILSSON (1969) for AVR.

When iron salts are used attention has also to be given to the risks of carry-over of iron ions also in the absence of chelating agents, particular as divalent iron and with suspended flocs rich in iron.

The selection of the precipitants also depends upon a number of factors, which to a considerable degree are related to national and local conditions and to the fact that phosphorus removal is usually not enough. Hence, a generally valid ranking list for the chemical cannot be made. Important items, which always ought to be considered in choosing the chemicals are:

1. The quality of the effluent and the remaining absolute amount of suspended solids phosphorus, nitrogen refractory and degradable organic matter, heaw metals, intestinal parasites and their eggs, bacteria, viruses and carry-over of the ions of the precipitants into the receiving water.

2. The costs of the chemicals, including even the costs for transportation, handling, storing and feeding and the costs, which may arise from corrosion and additional operations as sludge handling and disposal.

3. The evaluation must be based on the expected composition of unfiltered effluent samples. Filtered samples will give too good a performance for factors such as phosphate removal and carry-over of metal ions including the ions of the precipitants.

4. When sewage treatment including phosphate removal is introduced, phosphates from all sources are removed, and even if the detergent phosphates could be replaced completely, the phosphorus content of the water leaving a properly designed treatment plant would not be lower. The amount of precipitants would not be reduced in proportion to the decrease of the phosphorus content of the sewage. For Swedish conditions, for a replacement of two thirds of the detergent phosphates by NTA, one could only expect 15-20 per cent reduction of the amount of precipitant (AVR) needed to get the same phosphate content in the effluent and if the total quality of the effluent is considered, the difference might be nil or even an increase.

TREATMENT IN RURAL AND RESORT AREAS

In many rural areas it is possible, after removal of the solids, and usually after passage of the sewage through an anaerobic three-chamber cesspool, to infiltrate the

148 K.-A. NIELKERSSON

waste water directly into the soil. Phosphates are adsorbed on the soil and later used as a nutrient by the growing crop or general vegetation. In Sweden rules and recom- mendations have recently been issued by the National Swedish Environment ProductionBoard for the use and operation of these simple, inexpensive arrangements (SNV, 1971).

There are, however, places where this technique cannot be used because the ground does not absorb the phosphates or the infiltration technique cannot be used because of other reasons.

In many areas a more sophisticated treatment is then needed. Mini-sewage treatment plants in standardized units for a single house or a group of houses have been developed for such cases in Sweden. These units work with chemical-biological treatment or with chemical precipitation with e.g. aluminium based salts. The chemical units are less sensitive to uneven or interrupted load and they are therefore of a particular interest for resort houses. The units using chemical precipitation usually remove 90-95 per cent of the phosphorus and up to 90 per cent of the suspended solids and in units with biological and chemical purification about the same values are obtained for the reduction of suspended solids and phosphorus and the BOD-reduction is more than 90 per cent (ANONYMOUS, 1969, 1970; ULMGREN, 1970).

Tests are also being made in Sweden to carry out simple precipitation directly in cesspools with aluminium salts. This requires very little extra equipment in existing cesspools.

B R I E F D E S C R I P T I O N O F T H E

M A I N P R O C E S S E S U S E D I N M U N I C I P A L T R E A T M E N T P L A N T S

In FIGS. 2-6 the principle flow diagrams for the main processes used in municipal treatment plants are shown.

The processes described in FIGS. 2 and 3 include only mechanical and chemical steps. These processes could with advantage be applied to coastal regions and other situations, where the recipients have a high capacity of elimination of organic material in comparison with the amount of organic content of the effluent from the plant. Both processes have considerably lower investment costs than an activated sludge

Inflow Sandtrap

disintegrator

Chemicals

Flocculolion Sedimentation

or flotation

FIG. 2. Primary precipitation.

Outflow

Sludqe


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I - [ Flocculotion _1 Sedimentation - I or flotation

l

Outflow

I I

Sludge

Fla. 3. Secondary precipitation.

Inflow

Chemicals

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Aeration basin ~ J Sedimentation flotation or

i I I

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Sedimentation or

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Outflow

Excess stud_9.e - S 1 u d ~ _ _ ~

FiG. 4. Pre-precipitation.

Inflow Sandtrap = Sedimentation disintegrator

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__~ Aeration basin

Return sludge

_I Sedimentation ) o r

flotation I I

._~ E.~cess sludge

FIG. 5. Simultaneous precipitation.

Outflow

Sludge e.-


Inflow Sondtrap disintegrator Sedimentation

L Aeration I basin ]

Return sludge

_1 j Sedimentation

t LExcess sl_udge _ _Studge

Chemicals

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I -I f'a'a:'a° I k_

Outflow

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FIG. 6. Post-precipitation.

. . . . L J

plant, and with the primary precipitation the chemical step could be introduced quickly at an existing mechanical treatment plant. The secondary precipitation gives treated water of higher hygienic quality. Aluminium and ferric salts and lime could be used in these plants.

The primary and secondary precipitation is often called direct precipitation processes.

The pre-precipitation process, Fic. 4, could usually be applied quickly and with low additional costs to an existing treatment plant having mechanical and biological treatment steps. As precipitants, aluminium and ferric salts and lime can be used. However, sometimes the properties of the sewage have a considerable influence upon the performance and this has to be carefully evaluated, when the process is considered and applied. The process works smoothly, when the sewage is not too dilute and when the sewage does not contain too high a content of biologically degradable organic matter that is not removed in the pre-precipitation step, so that the carbon-phosphorus relationship will be too unfavourable in the biological step. However, experience from Stockholm show, that the organisms seem to adapt, so that they are not too sensitive to variations in the carbon-phosphorus ratio in the influent to the activated sludge section.

As 60-75 per cent of the organic matter is removed in the pre-precipitation step, the process also offers a possibility to quickly and at low investment cost introduce efficient chemical precipitation of phosphorus and at the same time to remove the over-load in a conventional plant, where the biological section is over-loaded. In this application of the process one usually only needs to install equipment for storing and dosing of the chemicals.

In a new plant a smaller biological section and less air is needed in comparison with conventional activated sludge plant having the same amount of incoming raw sewage.

With pre-precipitation the biological step is protected from toxic material such as heavy metals and dispersed oils, which are removed in the pre-precipitation step.

The pre-precipitation technique can also substantially improve purification fol-


lowing heavy rainfall, because the pre-precipitation step can tolerate considerable hydraulic overload and chemical pre-precipitation still removes organic and suspended matter and phosphorus from water that has to overflow from the pre-precipitation section directly into the receiving water. The pre-precipitation step functions then as a primary precipitation plant for the sewage that does not pass through the biological step. This has been demonstrated in full scale tests at Thalwil in Switzerland (STeNDAHL, 1971).

The total sludge volume is increased by 5-20 per cent depending upon the precipitants used and the operational conditions.

The process has been successfully applied to the Loudden, Eolshall and Akeshov sewage treatment plants in Stockholm, 30,000, 100,000 and 235,000 population equivalents respectively. By the introduction of the pre-precipitation technique more than 90 per cent phosphate removal was achieved and there was no need to extend the basin volumes of the overloaded Loudden plant; for the Akeshov plant it is expected that the plant, after introduction of pre-precipitation may handle 300,000 population equivalents without extending the original volume of the mechanical and biological sections.

The largest treatment plant in Stockholm, Henriksdal, 650,000 population equivalents is just being equipped for pre-precipitation.

AVR is used as precipitant. The thickened sludge has a solid content of 4.5-5.0 per cent after the introduction of pre-precipitation in comparison with 4.0-4.5 per cent before. After anaerobic digestion the solid content is ~ 3.0 per cent and after centri- fuging a solid content of 16-17 per cent is obtained.

The main characteristics of the principle of simultaneous precipitation are presented in FIG. 5. The concept of simultaneous precipitation is not a clearly defined process, but a chemical precipitant is added somewhere between the inlet to the outlet of the biological section or its recycle sludge in a biological sewage treatment plant, so that chemical precipita{ion takes place simultaneously with biological degradation of organic matter in the aerated, biological section. Usually the chemically loaded sludge from the outlet of the biological section is recycled as in the Swiss sludge recycle process of Thomas (THOMAS and RAI, 1970).

The simultaneous precipitation technique can usually be applied at low costs to existing biological plants except those which operate with trickling filters. Sometimes processes that have the characteristics of post-precipitation have been classified as simultaneous precipitation. This type of processes will not be called simultaneous precipitation here.

In simultaneous precipitation aluminium, ferrous and ferric salts are used. Extensive experimental work in full scale plants by AREGGER (1972) show, that with proper control of the feeding of the chemicals, there was no difference of phosphate removal efficiency on a molar basis between aluminium sulphate and ferric chloride.

Ferrous salts are either oxidized before they are added, or as practised mainly in Finland, directly added to the system as ferrous salts. In order to obtain reasonable operation the plant must be laid out and aeration provided in such a way that the excess iron is well transformed to the three-valent form.

The increase of total sludge volume is usually small in comparison with the sludge volume obtained when the same plant is operated as a conventional activated sludge plant.

152 K.-A. MEt.KEaSSO.~

The simultaneous precipitation processes are not without problems, if exposed to certain industrial wastes. They are more sensitive to hydraulic overload than pre- precipitation and they do not cope with storm water in heavy rainfall as well as pre-precipitation.

The suspended solid is usually considerably higher than for pre- and post-precipitation and the suspended matter can temporarily increase considerably with rainfall. The suspended solid can be a problem not only because suspended phosphorus enters the receiving waters but the receiving water may be sensitive to iron or a high suspended solid content may be objectionable on medical grounds.

Post-precipitation is demonstrated in FIG. 6. In post-precipitation the chemicals are added after the main biological section and chemical precipitation takes place in a final separate stage. The mechanical and biological steps may work with high loads and in many cases the primary clarifiers can be omitted. Many post-precipitation plants are in operation in Sweden.

A hybrid between post-precipitation and simultaneous precipitation, without primary clarifiers and with the main characteristics of post-precipitation is the two- stage biological-chemical Attizholz process. In this process 85 per cent or more of the organic matter is degraded in an activated sludge step, into which the raw sewage is fed directly. The chemicals are added to a second step, in which some further biological degradation proceeds but mainly chemical flocculation and precipitation take place. This not only removes the phosphorus but also the suspended organic matter from the first step efficiently.

Lime, aluminium and ferric salts are used as precipitants. Ferrous salts can be used but have to be oxidized to the ferric state before added. The majority of the Swedish post-precipitation plants use AVR.

The post-precipitation chemical sludge is low in organic matter. Post-precipitation processes give the best quality of the effluent, which should be

particularly considered, when high removal of phosphorus, highest hygienic performance and lowest content of organic material are wanted.

Chemical sludges usually are either stabilized together with the excess biological sludge in anaerobic digesters; in plants with less than 50,000 population equivalents aerobic stabilization of the chemical sludge alone or in mixture with the biologic sludges is used.

Phosphorus is usually not released by anaerobic digestion from sludges obtained with aluminium or iron salt precipitation. However, under certain circumstances phosphorus is released when the precipitation is made with ferric salts (MALuOTRA et at., 1971).

Sometimes the sludges are stabilized by the addition of quick lime. The combined biological + chemical sludge are dewatered after stabilization. In new plants in Sweden the dewatering is made in sieve band presses or by centrifuges.

Some new developments, which have been introduced in sewage treatment plants in Sweden are: a simple and reliable system for dosing and dissolving of solid chemicals based on a ftuidized bed technique, application of lamellar plate clarifiers for sedimentation of the chemical precipitants, usage of particular flotation technique for the separation of chemical flocs, in the flotation systems either part of the sewage water is oversaturated with air or the whole sewage water stream is oversaturated. Small air bubbles carry the flocs to the water surface, when the pressure is released.


PLANT PERFORMANCE AND EFFECT ON THE RECEIVING WATERS

The efficiencies of BOD- and P-removal of operating plants, expressed as per cent removal of the BOD and the total phosphorus content of the sewage, are summarized in TABLE 1.

TABLE 1. PtnUFICAa~ON RESULTS

Decrease of BOD Total P-removal o/ Type of treatment (/o) (~)

Mechanical sedimentation 30--40 5-15 Mechanical + chemical direct precipitation 60-70 ~ 90 Chemical + biological pre-precipitation 85-> 90 ~ 90 Biological + chemical simultaneous precipitation 80--90 75-85-(90) Biological + chemical post-precipitation > 90 > 90

For receiving waters, which have a large degradation capacity for organic matter, the primary and secondary precipitation processes often are sufficient. They give a phosphorus removal, that is as good or better than simultaneous precipitation plants in operation.

Most simultaneous precipitation plants run with a total phosphorus removal of 75-85 per cent (THOMAS and RAt, 1970); but in some cases the phosphate removal may periodically reach 90 per cent.

In general the pre-precipitation and post-precipitation processes give considerably higher BOD- and P-removal than simultaneous precipitation. The very best results are obtained with post-precipitation.

The post-precipitation hybrid of Attizholz operates with BOD- and P-removal of more than 90 per cent.

When the choice of a purification process has to be made, the best effluent quality in relationship to the receiving waters must be the deciding factor as long as the size of the investment in the purification plant is not prohibitive. Therefore, the absolute amounts of different undesirable impurities in the plant effluent are much more important than figures of percentage removal efficiency of parameters such as BOD and total phosphorus. Furthermore, percentage efficiencies often lead to false conclusions about the effect of sewage treatment processes, e.g. the common misconception, that the amount of phosphorus in the effluent of a plant is directly dependent upon the con- centration of phosphorus in the sewage times a percentage number, when the facts are, as they have been proved in many plants, that the phosphorus content of the effluent is independent of the phosphorus content of the sewage with proper control of the precipitation process. For example for Swedish conditions the experience is that a reduction of the phosphate of the detergents by 75-100 per cent will not lower the phosphate content of the effluent of a sewage treatment plant equipped with chemical precipitation.

It is unfortunate that in some countries the requirement of 80 per cent removal of phosphorus has been enforced because this concentrates interest on obtaining just 80 per cent reduction of phosphorus at a sewage treatment plant instead of con- centrating on the desirable effluent quality for the receiving waters at the lowest cost.

154 K.-A. MELKEKSSON

TABLE 2. PURIFICATION RESULTS IN FULL SCALE PLANTS

Effluent concn (mg -t) Chemical

Treatment added BOD SS Total P

None Primary treatment Convl. biological treatment Primary precipitation

Secondary precipitation

Simultaneous precipitation Pre-precipitation

Post-precipitation

150-250 200-300 8-12 100-175 75-125 7-11

15-30 20-30 4-9 AI or Fe 60-90 20--40 0.7-1.2

Ca 60-100 30-50 0.7-1.2 AI or Fe 5 0 - 9 0 1 0 - 3 0 0.6-0-8

Ca 50-90 3 0 - 5 0 0.6--0.8 AI or Fe 1 5 - 4 5 20-40 0,8-2-0 AI or Fe 1 0 - 3 0 10 -25 0.3-0.7

Ca 10-30 1 0 -3 5 0.5-0.7 AI or Fe 5-15 5-25 0"1-0"5

Ca 5-20 25-50 0.3-1.2

In TABLE 2, BOD, suspended solids (SS) and total phosphorus (P) obtained in operating plants are presented. I f the values of TABLE 2 are considered, one clearly realizes that the performances between the different processes are much larger, when emphasis is placed on effluent quality instead of on plant removal efficiencies.

The pre- and post-precipitation processes give the lowest phosphorus content of effluent. The suspended solids and BOD are very low for both these processes, lowest for post-precipitation. Even the hybrid post-precipitation of Attizholz is good in this respect (CEEP, 1971).

Simultaneous precipitation usually gives considerably higher values of total phosphorus, suspended solids and BOD in the effluent.

Good polishing of the effluent is particularly important when sewage treatment plants are large or where the impurities in the effluent are large in comparison with the acceptance levels of the receiving waters (CRONHOLM, 1968) or, as is often the case, when it is necessary to obtain high removal of heavy metals (NtLSSON, 1971), intestinal parasites and their eggs, bacteria and other pathogenic organisms (MELKERSSON et al., 1968 ; CHANDHURI and ENGELBRECHT, 1970; SPROUL et al., 1969).

Good polishing of the effluent also gives the lowest total phosphorus content and the lowest amount of other growth promotions in the effluent. This is important to attain the lowest possible algal growth potential of the effluent in the receiving waters. As a measure to achieve this the algal growth potential test (AGP-test) has been used at some plants in Sweden (FoRSBERG, 1972).

With the rapid increase in the number of sewage treatment plants in Finland, Sweden and Switzerland equipped with chemical precipitation which removes phosphorus efficiently regardless of its origin, including the phosphates in detergents, the trend of alarmingly increasing phosphorus loads entering lakes and rivers, which Vollenweider (VOLLENWEIDER, 1968) pointed out as a great danger for our waters, has been reversed in the early years of the 1970s in many critical areas in these countries (SNV, 1972). Sewage treatment with chemical precipitation has considerably improved the water quality of lake ZiJrich (THOMAS, 1971).

There are no good long term solutions for the utilization or disposal of the stabilized sludges. The sludges obtained by chemical precipitation have good fertilizing


properties, comparable to superphosphate, when used as a source of phosphorus. However, they contain also organic and inorganic impurities such as chlorinated hydrocarbons, heavy metals and patogenic organisms. Some of the heavy metals such as copper, zinc, molybdenum, manganese and iron are necessary trace elements, which nowadays are added to the fertilizers.

It seems reasonable and desirable that a long term solution ought to be developed, in which the sludges from the sewage treatment plants are recycled to agricultural usage. However, in order to do so, the sludges have to be pasteurized or in some other way safely and hygienically handled accordingly and the major part of undesirable heavy metals or organic substances controlled at the source, so that only minor or accidental amounts are collected in the sludges from sewage treatment plants. Enforcement of laws ought to make this possible during the present decade.

COST OF CONSTRUCTION AND OPERATING COSTS

In FXGS. 7-9 the construction costs of the plants and in FIGS. 10 and 11 the total annual operating costs of the plants including sludge handling but not sludge disposal are presented for plants in Finland, Sweden and Switzerland. All costs are given in native currencies in order to avoid the difficulties with changing exchange rates.

I000

9OO

800 - -

700 - -

T 6 0 0 -

-~ 5 0 0 - -

E 4 0 0 - -

3 0 0 -

i r 2 0 0

I00 -o 0

0

i03

o O0 0 0

0 0 0

2 3 4 5 6 7

Design number pe connected

iO 4

FIG. 7. Simultaneous precipitation in Finland. Construction costs of plants.

Many of the Finnish plants are of the extended aeration type. Usually they are small and the sewage flow per population equivalent is low. The Swiss and the Finnish plants included in the costs figures are usually old plants, which have been equipped later with facilities for chemical precipitation. Most of the Swedish plants have been built for chemical precipitation from the start. Direct comparison between costs are therefore difficult.

When old plants are equipped for pre-precipitation or simultaneous precipitation it is usually necessary to supplement the plant with equipment for storing, dosing and mixing the chemicals and for post-precipitation besides some additional tank volume.


I 0 0 0

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200

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Design number pe connecled

FIO. 8. Simultaneous precipitation (O) and hybrid post-precipitation (11) in Switzerland. Construction cost of plants.

1000 o

900 -

800 --

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u 400--

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FIO. 9. Post-precipitation in Sweden. Construction cost of plants.

In general the additional investment to an old plant is 3-15 per cent for pre-precipi- ration and simultaneous precipitation with aluminium or ferric salts and 8-20 per cent for post-precipitation.

When new plants are built and facilities for stabilization and dewatering equipment are included, the incremental costs of investments for chemical precipitation will usually be 5-20 per cent larger than the corresponding mechanical or biological sewage treatment plant. There are, however, cases, where the investment costs for a biological + chemical plant have been lower than for a conventional activated sludge process. An example of this is the hybrid post-precipitation plant of Ronneby, Sweden (CEEP, 1971).

In a new plant the incremental operating costs for chemical precipitation with aluminium and ferric salts are 25-45 per cent of the corresponding sewage treatment costs without chemical precipitation. For old plants with ample sludge stabilization


80

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6 0 - - T

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10--

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Fia. 10, Simultaneous precipitation in Finland. Total plant operating costs.

I00

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T ~, 7 0 - -

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FIG. 11, Post-precipitation in Sweden. Total plant operating costs.

and sludge handling facilities the incremental operating costs might even be lower, when the plants are supplemented with chemical precipitation in order to obtain a fair phosphorus removal quickly.

C O N C L U S I O N S

The removal of phosphorus by chemical precipitation is today feasible from the smallest units for a single house to very large sewage treatment plants. Many plants are in operation in areas where phosphorus removal is needed. Chemical precipitation can be introduced at existing sewage treatment plants in a short time and with available techniques,

Chemical precipitation removes phosphorus regardless of its origin. The concen- tration of the phosphorus in the effluent depends upon the process chosen but with proper control of the precipitant it usually is independent of the phosphorus content


of the incoming sewage. Chemical precipitation processes do not only remove phosphorus but with a proper choice o f process, heavy metals, organic matter, suspended solids, bacteria, viruses, intestinal parasites and their eggs can substantially be reduced in the effluent in compar i son with what usually is obtained in a biological or mechanical sewage treatment plant.

With the rapid increase o f sewage t reatment plants equipped with chemical precipitation, the supply o f phosphorus to lakes and rivers is already decreasing rapidly in Sweden and in many areas in Finland and Switzerland.

The costs for the chemical precipitat ion and efficient removal o f phosphorus o f the sewage treatment plants are very reasonable per person per day. For the disposal o f sludges f rom the sewage t reatment plants further research and development work is needed.

R E F E R E N C E S ANONYMOUS (1969) ByggnadsvSrlden 6/7, p. 142, p. 146. ANONYMOUS (1970) Testresultat for mini-verken. Teknik och milji~ 5, 42. AREGGER A. (1972) Gas- Wasser-Ab wasser 52, 75. CEEP (1971) Lespolyphosphates Coupables ou victimes ? p. 32. CHANDHURt M. and ENGELnREC~rr S. (1970) Removal of viruses from water by chemical coagulation

and flocculation. J. Am. Wat. Wks Ass. 563. CRONnOt.M M. (1968) Fosforreduktion medelst aluminium-sulfat vid Eolshfills reningsverk, Stockholm.

Vatten 2, 117. FORSBERG C. (1972) An algal assay procedure for monotoring sewage effluents. J. Wat. Pollut. Control

Fed. JENKINS D., FURGUSON J. F. and MENAR A. B. (1971) Chemical processes for phosphate removal.

Water Research 5, 369. MAL~OTRA S. K., PARmLO T. P. and HARTENSTEIN A. G. (1971) Anaerobic digestion of sludges

containing iron phosphates. J. sanit. Div. Proc. Am. Soc. Cir. Engrs 97, 269. MELKERSSON K.-A., NILSSON R. and STENOAHL K. (1968) Kemisk rening av avloppsvatten i biologisk

datum. Vatten 2, 132. NILSSON R. (1969) Phosphate separation in sewage treatment. Process Biochem. 4(5), 49. NILSSON R. (1971) Removal of metals by chemical treatment of municipal waste water. Water Research

5, 51. SNV (1971). Utredningar om fritidsbebyggelse, The Swedish National Environmental Protection

Board. Publication 1971 : 2. SNV (1972) Information From the National Swedish Environment Protection Board. SPROUL O., WARNER M., LAROCHELLE L. and BRUNNER D. (1969) IA WR-Conf, Prague. STENDAHL K. (1971) Private communication. THOMAS E. A. (1971) Oligotrophierung des Ziirichsees. Vierteljahrsschrift Naturforsch. Gesellschaft

116, 165. THOMAS E. A. and RAI H. (1970) Schweiz. Z. Gasversorgung Siedlungswasserwirtschaft 50, 179. ULMGREN L. (1971) Avloppsanl/iggningar f6r spridd bebyggelse. Hygienisk Revy 5, 224. VOLLENWEIDER R. A. (1968) Scientific fundamentals of eutrophication of lakes and flowing waters,

with particular reference to nitrogen and phosphorus as factors in eutrophication. OECD-Rep.

phosphorus in chemical and physical treatment processes

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