CHEMICAL PHOSPHOROUS REMOVAL AND ONLINE

ANALYSIS

This Presentation:

– Overview of Chemical Treatment Fundamentals – Chemical Control Strategy– Online Analyzer Evaluation

Statement of the ProblemControlling phosphorous discharged from municipal and industrial wastewater treatment plants is a key factor in preventing eutrophication of surface waters.

Phosphorous is one of the major nutrients contributing in the increased eutrophication of lakes and natural waters. Its presence causes many water quality problems:• increased purification costs• decreased recreational and conservation value of an

impoundment• loss of livestock and the possible lethal effect of algal

toxins on drinking water.

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 form.

– Orthophosphates: available for biological metabolism without further breakdown

– Polyphosphates: molecules with 2 or more phosphorous atoms, oxygen and in some cases hydrogen atoms combine in a complex molecule. Usually polyphosphates undergo hydrolysis and revert to the orthophosphate forms. This process is usually quite slow.

Chemical TreatmentChemical treatment for phosphorus removal involves the addition of metal salts to react with soluble phosphate to form solid precipitates that are removed by solids separation processes including clarification and filtration The most common metal salts used are :

• Alum (aluminum sulfate), sodium aluminate• Calcium (lime)

• Ferric chloride, ferric sulfate and ferrous chlorideChemical treatment is the most common method used for phosphorus removal to meet effluent concentrations below 1.0 mg/L.

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

Al3+ + HnPO43-n ↔ AlPO4 + nH+

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.

CalciumUsually 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 à 2CaCO3 ↓+ 2H2OAs 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 PO4

3- + 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. Neutralisation may be required to reduce pH before subsequent treatment or disposal. Re-carbonation with carbon dioxide (CO2) is used to lower the pH value.

Iron SaltsFerric chloride or sulphate and ferrous sulphate are all widely used for phosphorous removal, although the actual reactions are not fully understood. The basic reaction is:

Fe3+ + HnPO43-n ↔ FePO4 + nH+

Ferric ions combine to form ferric phosphate. They react slowly with the natural alkalinity and so a coagulant aid, such as lime, if added, would raise the pH and enhance the coagulation.

Chemical DoseThe required chemical dose is related to the liquid phosphorus concentration. For target concentrations above 2 mg/L (appropriate for chemical addition to a primary clarifier), a dose of 1.0 mole of aluminum or iron per mole of phosphorus is sufficient.

For lower phosphorus concentrations in the range of 0.3 – 1.0 mg/L, the dose can be in the range of 1.2 to 4.0 moles aluminum or iron per mole of phosphorus.

The pH value is an important factor for efficient removal of phosphorus using alum or other salts, as the solubility of their precipitates vary with pH. Phosphorus removal is most efficient in the pH range of 5 to 7 for alum and of 6.5 to 7.5 for ferric salts..

The first process is included in the general category of chemical precipitation processes. Phosphorous is removed with

90% efficiency and the final P concentration can be lower than 1.0 mg/l.

The coprecipitation 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.

The postprecipitation is 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. Postprecipitation gives also a good

removal of the SS that escape the final sedimentation of the secondary process.

Chemical Addition Pros and ConsPRO• – reliable• – low levels of P in effluent possible• – Retrofit for existing plant likely possible

CON• – cost of chemical feed system• – cost of chemicals• – substantial additional sludge production• – chemical sludge reuse or disposal may be more difficult• – may need to adjust pH

Sludge ProductionWith chemical addition, sludge production will increase in the wastewater treatment unit process where the chemical is applied. Sludge production has been noted to increase:

• by 40 percent in the primary treatment process • 26 percent in activated sludge plants

Chemical Dose DeterminationThe most important component of a control strategy for chemical phosphorous removal is the calculation of coagulant dosage. Dosage rates for aluminum salts or for iron salts are based on the molar ratio of available metal ion to phosphorous.

Theoretically to remove 1 mg/L of PO4-P you need• �9.6 mg/L of Alum• �5.2 mg/L of Ferric Chloride

Real life requires 0.5 to 15 times as much

Chemical Dose DeterminationJar Testing or Bench Testing – using a simple method to determine wastewater characteristics and real metal to phosphorus ratios required at different chemical injection locations.

Historical Trending – if historical data is sufficient to demonstrate hourly, daily or monthly phosphorus loading patterns, the varying chemical dosing levels can be configured via with the plant’s SCADA system using these trends.

Third party laboratory analysis – data used for reporting purposes. This data can also be used to fine-tune chemical dosage, but the long wait times are a problem for “real time” control. Increasing analysis costs are another concern if multiple sample locations are monitored.

Plant’s lab analysis – chemical dosage is adjusted based on the data collected and analyzed in laboratory by the plant staff. This is a labor consuming process and difficult for the “real time” process adjustments.

Chemical Dose DeterminationProper control is difficult to achieve using manual techniques such as grab samples and periodic jar tests. There are several reasons for this difficulty:• incoming phosphate concentrations can vary in unpredictable ways as a

result of industrial contributions.• incoming phosphate concentrations are RARELY in proportion to flow• conversion of polyphosphate to orthophosphate prior to coagulant addition

will affect coagulation efficiency. Process conditions, particularly pH and temperature, can significantly influence polyphosphate conversion.

• if reclaimed products such as pickle liquor are used as a coagulant, the concentration of available metal ion will also be variable. This will result in a highly variable phosphate coagulation rate and, in the absence of on-line monitoring, will require frequent manual adjustments to avoid overfeed or underfeed.

Chemical Dose DeterminationInsufficient coagulant dosages can produce an effluent with excessive turbidity, but excessive coagulant dosage can also produce the same result. Surplus coagulants can also have an adverse effect on disinfection processes, by exerting an oxidation demand. Surplus metal salts can coat ultraviolet disinfection tube surfaces. Thus, it is important that the process be well controlled.

Solids SeparationAs effluent permit limits for phosphorus are commonly expressed as total phosphorus, precipitation of the soluble phosphorus (orthophosphates) into a particulate form is only a part of the job.

The solids separation step (clarification and/or filtration) must be capable of removing the effluent TSS. The more TSS removed the lower concentration of Total Phosphorous.

Monitoring Strategy The monitoring strategy will depend on the chemical addition point and on the control strategy to be employed.

• Orthophosphate should be monitored at frequent timed intervals from a point in the process prior to coagulant addition. The sample point should be after the point at which polyphosphate has been converted to orthophosphate. This is usually not at the headworks, but after primary clarification or immediately after influent addition to the aeration tanks. Filtration of suspended solids may be required prior to analysis, but should not affect accuracy, since the soluble phosphate is the parameter of interest.

• A sample point following coagulation can be selected to monitor the efficacy of the coagulation process. The sample point may be the effluent from a final clarifier, or effluent filtration. Orthophosphate measured at this sample point represents the fraction of dissolved phosphate that remains after coagulation. Typically 60-80% of the total phosphorous.

A PROPOSED SOLUTION – ONLINE ORTHO PHOSPHATE ANALYZER

– Online Phosphorus Analyzer• Real time• Continuous monitoring• SCADA-linked for automatic dosing control• Flexible for “feed forward” or “feed back” control• Assure Permit Compliance

– Key – Chemical Saving

Ortho-P vs. Total-P – Total Phosphorus

• Organic portion• inorganic portion

– Why measure ortho-P in process control• Ortho-P is dissolved portion • Chemical precipitation removes Ortho-P• Relationship between ortho-P and Total-P

– Clarifier or filtration performance is important

Phosphorus Analyzer Selection

– Check References• Confirm reliability• Maintenance requirements

– Interference compensation– Self-cleaning and self-zeroing

• Prevents optical cell fouling• Self –zeroing restores background

Phosphorus Analyzer Selection

– Sample handling• High turbidity samples may require filtering• Filter cleaning requirements

– Maintenance requirement• Weekly, monthly, annual

– Overall cost of ownership• Cost of reagents, spare parts, cleaning

chemicals

Effluent SamplingEffluent Sampler Effluent Reuse or (W3)

On Line AnalysisIn general, phosphate analyzers are the complex integrations of chemical, electrical and mechanical technologies:

– Labor saving– Speed of data acquisition– Consistency, reliability and accuracy

But saving labor and acquiring data are not justified if the online analyzer only adds a maintenance burden or if the analyzer is not reliable.

Installations & Case HistoriesSheboygan, Wisconsin

– WWTP servicing population of 50,000 - EBPR– Current phosphorus limit was 1.0 mg/l – Ferric chloride spending $160,000 annual at the 1.0 mg/L

limit– First online phosphate analyzer installed in 2010– Second online phosphate analyzer installed in 2012– data “feed back” to SCADA for ferric chloride dosing

control– Chemical savings $$$ paid for analyzer in 4 months.

Installations & Case Histories

New London, Wisconsin• 2 MGD design activated sludge process plant• Phosphorus permit limit 1.0 mg/l• Online analyzer data “feed back” direct to

chemical feed pump for ferric chloride dosing control

• $900 per month chemical expense saving

Installations & Case Histories

– Kiel, Wisconsin• 0.9 MGD activated sludge process• Phosphorus permit limit 1.0 mg/l• A cheese packaging factory is the major

contribution source• Online analyzer data “feed back” to SCADA for

ferrous sulfate dosing• 20% chemical expense saving

In ConclusionMeeting tightening discharge limitation with chemical phosphorous removal can require:

• Quick response to concentration changes• Respond to process changes• Chemical cost savings• Reduce of maintenance burden