SUSTAINING PHOSPHORUS REMOVAL IN WRRFs RELIABLY – THE CHEMICAL WAY AND ITS FEATURES

When limits are placed on P, it is usually for the parameter Total Phosphorus (TP). But the parameter is composed of other sub species as the following:

Though TP measurement is simple and cost effective, the disadvantage is that certain sub species have varying environmental impacts which is unaccounted for [1].   

Phosphorus can enter a WRRF in the following forms:

1. Organic phosphorus (found in organic matter and cell protoplasm)
2. Polyphosphate (commonly found in detergents)
3. Soluble inorganic orthophosphate (typically constitutes 50 – 80% of the TP)

During biological treatment, most of the organic phosphorus (if biodegradable) and polyphosphate are converted to inorganic orthophosphate. Some of the available orthophosphate is assimilated and incorporated into biomass required for growth (BOD: TP ratio of 100:1).

Usually, the organic phosphorus is the smallest fraction of the TP. The orthophosphate is the most readily available form for biological and / or chemical removal (precipitation via metal salts) [7]. 

When stringent TP regulations are in place, chemical addition is always present. The following location(s) are preferred for chemical addition:

After chemical addition, the goal is to capture the soluble reactive P (mainly orthophosphate) in a particulate form via solids separation processes (sedimentation, filtration or membrane separation). Some of the colloidal P fraction is also removed here.

An advantage provided by chemical precipitation (specifically for WRRFs not practicing EBPR) is that the P fraction removed remains in the wasted sludge (from biological process) as an inorganic precipitate. It is not released during biosolids storage, thickening or Anaerobic Digestion (AD) process [6].

Al Vs Fe METAL SALTS

When chemicals are being considered for P removal, there is an operational cost associated with the chosen chemical and its dosage. Therefore, from a process economy viewpoint, the chemical feed system design must consider:

1. Minimum and maximum ambient or room temperature
2. Minimum, average, and maximum wastewater flows
3. Minimum, average, and maximum anticipated dosages [6]

With chemical addition, it is principally the soluble orthophosphate ions that will be removed. The most common metal salts used for P removal are:

1.     Aluminum (Al) based

2.     Iron (Fe) based

Between both, Al has a simpler chemistry as it is not considered under dangerous goods category unlike Fe. Also, with Fe salts, there will be a fast orthophosphate removal phase followed by a slow phase. But with Al, orthophosphate removal is more instantaneous [1].

Alongside Fe dosing for P removal, if the WRRF treatment scheme has ADs, owing to low solubility of ferrous sulfide, ferric salts are also added for the dual purpose of controlling hydrogen sulfide and odors. Though this might seem to be a perceived advantage, at higher doses, it can lead to scale formation in AD piping [6].

Based on stoichiometric calculations, using alum will produce less sludge than Fe salts but studies have shown that alum sludge can be more difficult to concentrate and dewater [5]. In addition, ferric could also serve as an odor control option in preliminary or primary treatment process within WRRF making it a primary choice for chemical P removal too [6].

When the wasted sludge from WRRF is hauled away and applied as a soil amendment, presence of Fe within is of more value than Al. However, if the WRRF is utilizing Ultraviolet (UV) disinfection, using Fe salts will have a negative influence on UV transmittance as compared to Al salts [6].

TAKEAWAY FOR DESIGN ENGINEERS

Irrespective of the chosen metal salt and its dosage, site specific wastewater characteristics (COD, TSS), competing substances (eg: sulphide), method of chemical addition, dosage locations, reaction pH, alkalinity, initial mixing conditions, flocculation method and time after chemical addition are important parameters that must be considered. Ultimately, these factors will affect the relationship between the dosage and the P removal efficiency.

MECHANISM OF P REMOVAL

Earlier, the chemical P removal design was based on equilibrium precipitation theory. However, in 2008, following the publication of two research papers, the engineering community’s understanding of chemical P removal improved further.

Therefore, whether you choose to dose Al and / or Fe, the next thing to know is what’s happening immediately after dosing. That’s what we look at next.

A SHORT STORY

To state things simplistically, the P removal mechanism is as follows:

Step 1: When added, Al and / or Fe salts form metal hydroxides (HMOs).

Step 2: Orthophosphate is removed via co-precipitation and adsorption (also known as complexation) on HMO surface. 

Step 3: With passing time, HMO flocs age which reduces available sites for orthophosphate complexation. In short, P removal efficiency reduces.

Step 4: Additional reaction takes place if the environment is right (mixing, pH, wastewater characteristics, absence of chelating agents etc) [2].

THE LONGER STORY

When chemicals are added, there is instantaneous formation of HMOs. These HMOs have a high number of active surface sites for interaction with orthophosphate. This orthophosphate then co-precipitates with HMO-high which are tracked in process simulators. Majority of the orthophosphate removal occurs in this phase [4].  

With passing time, the formed flocs begin to change characteristics. It’s not that the orthophosphate does not adsorb to the HMO but the adsorption rate is lowered than as seen above. Essentially, the flocs lose the potential orthophosphate adsorption sites. These sites are tracked in the process simulators as HMO-low [4].

With some more passing time, the HMO-low sites also begin to change into an aged species that cannot take up any remaining orthophosphate. This is also tracked within the process simulator models [4].

Alongside HMO-high which, with time, transform to HMO-low, there are also unbound HMO which represent additional potential sites for orthophosphate uptake. These unbound HMO also have high and low adsorption sites. To create these unbound HMO sites, metal dosing must be increased and complimented with good mixing and contact time. With time, (1) these unbound sites fill up with adsorbed orthophosphate and (2) age out [4].

STORIES SUMMARY

To reiterate, there is an initial fast removal phase (majority of the orthophosphate removal occurs via co-precipitation in duration of few minutes) and a slow removal phase (occurring via adsorption lasting hours and days). The slow removal phase can be utilized better in practice by providing good mixing conditions and chemical solids recycling rather than indiscriminately dosing chemicals (which translates to higher chemical costs and sludge production). Solids recycling helps to increase P removal % via adsorption on left over HMO sites [2].

When longer contact time is factored in design (both SRT and HRT), for interaction between orthophosphate and HMO, it mitigates effects of inefficient mixing (especially in biological treatment configurations).

DOSING CALCULATIONS AND RELATED CONSIDERATIONS

There are two ways to determine the chemical dosages required for orthophosphate removal:

1. Hand calculations (based on stoichiometry)
2. Using commercial process simulators (based on HMO precipitation)

For hand calculations, metal dose in mole and its equivalent ppm needs to be accounted for. In addition, there are two types of precipitants that are formed that become the total chemical sludge produced from dosing.

For the graphs shown above, remember the following:

1. The molar ratio of the metal dose required is for the soluble portion of P or orthophosphate.
2. When lower residual soluble phosphorus is required for the WRRF (<0.1 ppm), it is recommended to carry out bench or pilot scale testing. That way, the actual molar ratio required to reach lower effluent soluble P will be more accurate. Even jar testing is limited and cannot be well simulated [6].
3. Though % P removal increases, and effluent P concentration reduces with increasing metal salts molar dosage, the incremental removal observed diminishes [5].

Stoichiometric calculations are approximate as the orthophosphate precipitation with Al or Fe salts is a complex process involving co-precipitation and adsorption.

Process simulators are better at tracking actual orthophosphate removal through the unit operations and processes of WRRFs. Mixing conditions (G value) within reactors, contact time and solids recycling are better accounted for in process models owing to which chemical dosages required to achieve the desired P levels are more optimized as compared to hand calculations [1].

Process simulators also consider biological assimilation, EBPR process, metal salts reduction and precipitation, orthophosphate precipitation. They also have a factor assigned for high and low HMO sites and depending on the process modeler’s user proficiency and experience, the co-precipitation and adsorption rates can be adjusted [1].

MIXING MATTERS

Mixing intensity (measured as G or velocity gradient) is an important design and operational parameter for chemical P removal. With good mixing, it is possible to:

1. Achieve better initial contact between the dosed metal salt (coagulant) and the orthophosphate [5].
2. Reduce coagulant dosing by 50 – 75% [1].
3. Improve complexation efficiency [2].
4. Break the flocs up. When the metal hydroxide flocs are smaller, the available reactive sites are higher, and this helps to reduce the floc aging process.

Within WRRFs, flow distribution structures are good dosing locations for providing a flocculating environment. Other ideal dosing locations include weirs, tanks with mechanical mixing, aeration diffusers etc.

As an example, see below:

DESIGN TIP: The G value indicated above are not practical design values. A G value of 200 (per second) is more realistic and recommended. Within WRRFs, G values ranging between 20 – 100 (per second) at the dosage point is poor.

At the point of dosing, when the mixing intensity is higher, it will lead to better contact between the added metal and the orthophosphate [3]. As the HMO sites are more readily available under rapid mixing conditions, this usually translates to instantaneous removal and lower orthophosphate concentration in the treated effluent. It is recommended to have a rapid mixing time of 10 – 30 seconds [5].

After rapid mixing at dosage point, it must be followed up with gentle mixing conditions for flocculation to occur. Though liquid movement through the WRRF is sufficient for flocs formation prior to solids separation, insufficient flocculation time, aggressive pumping and aeration conditions can disrupt floc formation [5].

It is recommended to provide 15 – 20 minutes of flocculation time as it helps with particle agglomeration. As the flocs size increase, the available adsorption sites within are reduced. This is another reason mixing is important as it helps to break up the bigger flocs, maintain smaller active HMO sites and reduce the aging process. In the slower mixing phase, the flocs enter an ‘aging’ process.

As part of co-precipitation where flocculation can occur in aeration basins or channels preceding Secondary Clarifiers (SCs), dedicated flocculation zones are still recommended in SCs. This provides flexibility in achieving control over G values for flocculation.  

Dosing organic polymers (ideally using progressive cavity pumps) at a sufficient downstream distance from metal salts and with adequate mixing is effective as it helps:

1. To achieve lower orthophosphate concentration (on its own, polymers do no remove or precipitate orthophosphate).
2. Improve solids separation processes by increasing the size and compactness of the biological and chemicals solids. As a result, the settling velocity increases which is desirable in clarifiers.

BENEFITS OF MULTIPLE LOCATIONS DOSING

As the residual orthophosphate concentration reduces through the unit operations and processes within WRRFs, the rate of co-precipitation and adsorption reduces as well. Therefore, adding metal salts at upstream of WRRF is more beneficial than adding them towards the downstream end only. This is why, multiple dosing locations are advocated as:

• The chemical dosages and the associated sludge production is lower.
• Higher P removal levels can be observed.

Based on the above snapshot (pre-precipitation and co-precipitation), though it appears that the two-step dosing is more beneficial, the calculations haven’t considered solids recycling (especially in an Activated Sludge Process configuration). In the wasted sludge from ASP, the HMO – low and the HMO – high are lower as compared to primary sludge.

This is why, process simulators > hand calculations.

In terms of design approach effectiveness for achieving lower TP limits, bench / pilot scale testing > process simulators > hand calculations.

Also, irrespective of the metal salt type, for multiple dosing, consider the following in design:

1. P removed biologically.
2. Potential solubilization of particulate P [6].

THE COMPETITORS

Presence of higher Chemical Oxygen Demand (COD), alkalinity, and Total Suspended Solids (TSS) in the influent can impact the residual orthophosphate levels in the treated effluent [3,5].

The metal hydroxide precipitation is affected when the bicarbonate ions are more than the orthophosphate concentration as they compete for active HMO adsorption sites. 

Within the organic matter, the carboxylic and phenolic compounds present can compete for binding sites on the HMO surface [3]. Presence of colloidal COD means competition for active surface sites on HMO ultimately lowering orthophosphate removal [4]. Especially for biological processes, the competition is also influenced by mixing at the dosing point.  

Al and Fe ions can react with humic and fulvic acid substances which will form insoluble complexes with the dosed chemical ions and mineral oxides. This too eventually reduces the reactive sites for orthophosphate precipitation. As a result, the dosage needs to increase to create new unbound HMO sites.

When alum is added before Primary Clarifiers (PCs), along with organics and TSS removal, the soluble phosphorus competes for available alum. Therefore, it increases the dosage required to obtain the desired orthophosphate concentration.

Similarly, if the WRRF influent has significant levels of sulfide, Fe dosage will be higher in PCs to achieve the desired orthophosphate concentration. This is owing to the competing hydroxide and sulfide reaction.

PAY ATTENTION TO pH AND ALKALINITY

Both Al and Fe are acidic in nature. Upon dosing, the system will be impacted. Carrying out an alkalinity balance is an essential activity as part of the chemical P removal design.

Broadly, a pH range of 5.5 – 7 is most suitable for both Al and Fe salts. As the pH increases, the orthophosphate removal efficiency drops. At the dosing point, if the pH is in the range of 5.5 – 7, thereafter it does not play a key role in co-precipitation and coagulation. Therefore, the choice of chemical will depend on pricing, availability, and sludge handling [3].

The reason excess chemical dose is applied (in addition to the stoichiometric dose) is to reduce the pH in the desired range as above and for the formation of metal hydroxides [7].  

As per [8], at pH around 6, the P removal efficiency by alum was most optimum. Good efficiency with alum was also observed between pH 5 – 8. However, ferric was more efficient than alum at pH below 6.

Within the suitable pH range, HMO formation is dominant. If the pH drops further below, they resolubilize to soluble metals which is undesirable and metal hydroxide precipitation is limited. At higher pH, the metal salts form other species [Me (OH)4] which again is undesirable [1].

SOLIDS SEPARATION, A KEY ELEMENT

The P that enters a WRRF must be converted to a solid form to be removed from the wastewater. Therefore, the P either exits the WRRF in the effluent or the solids stream. For sedimentation / filtration / membrane filtration design, you must consider:

• The solids removal efficiency
• P content of the solids
• Orthophosphate concentration

We have already established that to meet strict TP concentration in the treated WRRF effluent, the metal salt dosage will be higher. As a result of the higher dosage, it translates to a metal hydroxide complex with solids particles containing orthophosphate. These solids will have a lower P concentration as compared to solids produced with a lower metal salt dosage. The lower P content in the solids helps to achieve stringent TP limits in the liquid stream.

If stringent TP limits are in place, the chosen solids separation process must be highly efficient to produce low or non-detectable TSS concentration. In addition, the chosen process must be able to handle higher metal salt dosages to achieve stringent TP limits [8].