ch2mbr

Phosphorous Removal from Wastewater

Controlling phosphorous discharged from municipal and industrial wastewater treatment plants is a key factor
in preventing eutrophication of surface waters. Phosphorous, municipal wastewaters
may contain from 5 to 20 mg/l of total phosphorous, of which 1-5 mg/l, is organic and the rest in inorganic.

Phosphorous removal processes basically involve two methods: 1) biological and 2) physical/chemical treatment.
The biological removal of phosphorus is achieved with the following processes:

Phosphate removal is currently achieved largely by chemical precipitation, which is expensive and causes
an increase of sludge volume by up to 40%. An alternative is the enhanced biological phosphate removal (EBPR).

1. Chemical Precipitation

Chemical precipitation is used to remove the inorganic forms of phosphate by the addition of a coagulant and a mixing of wastewater
and coagulant. The multivalent metal ions most commonly used are calcium, aluminium and iron.

1-1 Calcium

It is usually added in the form of lime Ca(OH)2. It reacts with the natural alkalinity in the wastewater
to produce calcium carbonate, which is primarily responsible for enhancing SS removal.
Ca(HCO3)2 + Ca(OH)2 ? 2CaCO3 ↓+ 2H2O

As the pH value of the wastewater increases beyond about 10, excess calcium
ions will then react with the phosphate, to precipitate in hydroxylapatite:

10 Ca2+ + 6 PO43- + 2 OH- ↔ Ca10(PO4)*6(OH)2 ↓

Because the reaction is between the lime and the alkalinity of the wastewater, the quantity required will be,
in general, independent of the amount of phosphate present. It will depend primarily on the alkalinity of the wastewater.

The lime dose required can be approximated at 1.5 times the alkalinity as CaCO3. Neutralization may be required to reduce
pH before subsequent treatment or disposal. Re-carbonation with carbon dioxide (CO2) is used to lower the pH value.

1-2 Aluminium and Iron:

Alum or hydrated aluminium sulphate is widely used precipitating phosphates and aluminium phosphates (AlPO4). The basic reaction is:

Al3+ + HnPO43-n ↔ AlPO4 + nH+

This reaction is deceptively simple and must be considered in light of the many competing reactions and their
associated equilibrium constants and the effects of alkalinity, pH, trace elements found in wastewater.

The dosage rate required is a function of the phosphorous removal required. The efficiency of coagulation falls as the concentration
of phosphorous decreases. In practice, an 80-90% removal rate is achieved at coagulant dosage rates between 50 and 200 mg/l.

Dosages are generally established on the basis of bench-scale tests and occasionally by full-scale tests,
especially if polymers are used. Aluminium coagulants can adversely affect
the microbial population in activated sludge, especially protozoa and rotifers, at dosage rates higher than 150 mg/l.

However this does not affect much either BOD or TSS removal, as the clarification function of protozoa
and rotifers is largely compensated by the enhanced removal of SS by chemical precipitation.

Ferric chloride or sulphate and ferrous sulphate also know as copperas, are all widely used for phosphorous removal,
although the actual reactions are not fully understood. The basic reaction is:

Fe3+ + HnPO43-n ↔ FePO4 + nH+

1-3 Strategies

The main phosphate removal processes are (see picture below):

·  The first process is included in the general category of chemical precipitation processes.
    Phosphorous is removed with 90% efficiency and the final P concentration is lower than 0.5 mg/l.
    The chemical dosage for P removal is the same as the dosage needed for BOD and SS removal, which uses the main part of these chemicals.
    As mentioned above lime consumption is dependent on the alkalinity of the wastewater: only 10% of the lime fed
    is used in the phosphorous removal reaction. The remaining amount reacts with water alkalinity, with softening.
    To determine the lime quantity needed it is possible to use diagrams: i.e. the lime used to reach ph 11 is 2-2.5 times water alkalinity.

·  The post-precipitation is a standard treatment of a secondary effluent,
    usually using only metallic reagents. It is the process that gives the highest efficiency in phosphorous removal.
    Efficiency can reach 95%, and P concentration in the effluent can be lower than 0.5 mg/l.
    Post-precipitation gives also a good removal of the SS that escape the final sedimentation of the secondary process.
    Its advantage is also to guarantee purification efficiency at a certain extent even if the biological process
    is not efficient for some reason. The chemical action is stronger, since the previous biologic treatment transforms part of the organic
    phosphates in orthophosphates. Disadvantages are high costs for the treatment plant (big ponds and mixing devices)
    and sometimes a too dilute effluent. Using ferric salts there is also the risk of having some iron in the effluent,
    with residual coloration. The metallic ions dosage is about 1.5-2.5 ions for every phosphorus ion (on average about 10-30 g/mc of water).

·  The co-precipitation process is particularly suitable for active sludge plants,
    where the chemicals are fed directly in the aeration tank or before it. The continuous sludge recirculation,
    together with the coagulation-flocculation and adsorption process
    due to active sludge, allows a reduction in chemical consumption. Moreover the costs for the plant are lower,
    since there is no need for big post-precipitation ponds. In this process the chemical added are only iron and aluminium,
    lime is added only for pH correction. Lower costs and more simplicity are contrasted by a phosphorous removal
    efficiency lower than with post-precipitation (below 85%). The phosphorous concentration in the final effluent
    is about 1 mg/l. Another disadvantage is that biological and chemical sludge are mixed,
    so they cannot be used separately in next stages. Mixed sludges need bigger sedimentation tanks than activated sludge.

2. Biological Processes

2-1 P Removal Only

·  A/O Process
·  PhoStrip Process
·  Enhanced Biological Phosphorus Removal (EBPR)

2-2 P & N Removal Simultaneously

·  A2O
·  Bardenpho
·  MUCT
·  VIP
·  SBR
·  CH2-MBR

Over the past 20 years, several biological suspended growth process configurations have been used to
accomplish biological phosphorous removal. The most important are shown in the following picture.

The principal advantages of biological phosphorous removal are reduced chemical
costs and less sludge production as compared to chemical precipitation.

In the biological removal of phosphorous, the phosphorous in the influent wastewater is incorporated into cell biomass,
which is subsequently removed from the process as a result of sludge wasting.
The reactor configuration provides the P accumulating organisms (PAO) with a competitive advantage over other bacteria.
So PAO are encouraged to grow and consume phosphorous. The reactor configuration in comprised of an anaerobic
tank and an activated sludge activated tank. The retention time in the anaerobic tank is about 0.50
to 1.00 hours and its contents are mixed to provide contact with the return activated sludge and influent wastewater.

2-2-1 In the anaerobic zone

Under anaerobic conditions, PAO assimilate fermentation products (i.e. volatile fatty acids)
into storage products within the cells with the concomitant release of phosphorous from stored polyphosphates.
Acetate is produced by fermentation of bsCOD, which is dissolved degradable organic material that can be easily
assimilated by the biomass. Using energy available from stored polyphosphates,
the PAO assimilate acetate and produce intracellular polyhydroxybutyrate (PHB) storage products.
Concurrent with the acetate uptake is the release of orthophosphates, as well as magnesium, potassium,
calcium cations. The PHB content in the PAO increases as the polyphosphate decreases.

2-2-2 In the aerobic zone

Energy is produced by the oxidation of storage products and polyphosphate storage within the cell increases.
Stored PHB is metabolized, providing energy from oxidation and carbon for new cell growth. Some glycogen
is produced from PHB metabolism. The energy released from PHB oxidation is used to form polyphosphate bonds in cell storage.
The soluble orthophosphate is removed from solution and incorporated into polyphosphates within the bacterial cell.
PHB utilization also enhances cell growth and this new biomass with high polyphosphate storage accounts for
phosphorous removal. As a portion of the biomass is wasted, the stored phosphorous
is removed from the bio-treatment reactor for ultimate disposal with the waste sludge.

The amount of phosphorous removed by biological storage can be estimated from the amount of bs-COD
that is available in the wastewater influent. Better performance for BPR systems is achieved when bs-COD acetate is available at a steady rate.

2-2-3 Control Issues

The most important component of a control strategy for biological phosphorous removal is to assure that soluble phosphorous is being released in the anaerobic zone and is being removed in the aerobic zone. Proper control is difficult to achieve using manual techniques such as grab samples and DO monitoring. There are several reasons for this difficulty: ·  Incoming phosphate concentrations can vary in unpredictable ways as a result of industrial contributions.
·  Incoming phosphate concentrations do not necessarily vary in proportion to flow.
·  Long acclimation periods are required to cultivate
    a population of organisms that can survive alternating anaerobic and aerobic cycles.
·  High recycle nitrate concentrations can inhibit anaerobic
    zone processes. Thus, DO can be low but the anaerobic zone can be inhibited for other reasons.
·  DO is not the best indicator of aerobic zone phosphorous removal
    performance. Reduction of soluble phosphate is the most direct and most reliable indicator.