Unlocking Biological Efficiency through the Strategic Application of Micronutrients in Activated Sludge Systems

In industrial and municipal wastewater treatment, activated sludge processes are often considered the workhorses of secondary purification.   For many, the default is to assume that optimal biological performance can be maintained solely by the rigorous monitoring and control of a system’s macronutrients.  More specifically, maintaining Carbon (BOD), Nitrogen (N) and Phosphorous (P) at ratio concentrations of 100:5:1 respectively.  In reality, it is naïve to think that bacteria health can be maintained solely on the basis of proper C:N:P control.  It is with that kind of singular focus that operational vulnerabilities start to become apparent.

In addition to macros, “Micronutrients” need to be routinely assessed and maintained.  When present in sufficient quantity, these materials, typically in the form of trace metals, get used to synthesize enzymatic pathways for nitrifying, denitrifying, and heterotrophic bacteria, and promote the formation of stable/highly settleable floc.  If deficient in any way, nitrification/de-nitrification will tail off, floc formation will suffer, filamentous bulking becomes prevalent, BOD removal efficiency drops and dewatering applications start to see a notable increase in polymer demand.

Given the above, it is easy to see that micronutrient monitoring and control is a critical requirement.  Especially when looking to maximize operational performance.

Micronutrients and the Biological Roles they fulfill

When properly maintained, micronutrients can perform as a catalyst to ignite metabolic processes and promote the formation of healthy cellular structures.  They can also support the production of extracellular polymeric substances (EPS), which is the biological glue that binds individual bacteria together to form robust floc structures.  The following is a list of micronutrients that are highly effective in supporting these and similar high-priority activities.

• Iron (Fe) – In terms of biochemical function, iron is the cornerstone of cellular respiration. It is a core component of cytochromes, iron-sulfur proteins (Fe-S clusters), and catalase enzymes which are responsible for electron transport and oxidative stress defence. It also acts as a structural bridging agent, by promoting an attraction amongst negatively charged functional groups on the cell walls of bacteria (floc formation).  Target concentration in mixed liquor – 0.5-2.0 mg/L
• Nickel (Ni) – Is a mandatory cofactor for key microbial enzymes, most notably urease (hydrolyzes urea to ammonia for bacterial uptake) and hydrogenases. In anaerobic sections or deep zones of an activated sludge system, nickel is considered a vital component for methanogenic pathways and specific structural enzyme configurations. Target concentration in mixed liquor – 0.05 – 2 mg/L.
• Cobalt (Co) – Serves as the central atom in cobalamin (Vitamin B₁₂) coenzymes. These enzymes as catalysts for critical carbon-segment transfers, nucleic acid synthesis, and fatty acid metabolism within heterotrophic biomass. Without cobalt, cell division will slow which can lead to the formation of fragile, low-density floc. Target mixed liquor concentration – 0.01-0.05 mg/L.
• Molybdenum (Mo) – is a crucial structural component in the formation of nitrate reductase, the primary enzyme responsible for converting nitrate (NO₃^–) to nitrite (NO₂^–) during denitrification. Target concentration in mixed liquor – 0.05 – 0.1 mg/L.
• Zinc (Zn) – Zinc plays both a catalytic and structural role in hundreds of bacterial proteins. It stabilizes the tertiary structure of DNA-binding proteins (“zinc fingers”) and activates crucial enzymes like carbonic anhydrase and various dehydrogenases responsible for organic substrate degradation. Target concentration in mixed liquor – 0.1 – 0.5 mg/L.
• Copper (Cu) – Is required to form cytochrome-c-oxidase, a terminal enzyme complex in the respiratory electron transport chain that reduces molecular oxygen in water so organic carbon can be metabolized by the biomass. Target concentration in mixed liquor – 0.02 – 0.1 mg/L.

Chemistries that are used to restore Micronutrient Imbalances

When an ICP mass spectrometry analysis of the mixed liquor identifies a deficiency, targeted chemistry must be applied to correct that imbalance.

Many treatment facilities will apply high-purity inorganic salts because they are low cost and quickly dissolve when added.  Some of the more additives in use include:

• Iron – Ferrous Sulfate Heptahydrate (FeSO₄ – 7H₂O) or Ferric chloride (FeCl₃).
• Nickel – Nickel Sulfate Hexahydrate (NiSO₄ – 6H₂O).
• Cobalt – Cobalt Chloride Hexahydrate (CoCl₂ – 6H₂O).
• Molybdenum – Sodium Molybdate Dihydrate (Na₂MoO₄ – 2H₂O).
• Zinc – Zinc Sulfate Heptahydrate (ZnSO₄ – 7H₂O).
• Copper – Copper Sulfate Pentahydrate (CuSO₄ – 5H₂O).

If wastewater is high in pH/alkalinity or elevated sulfide residuals are present, inorganic salts can get consumed and precipitate out of solution as metal hydroxides or sulfides.  When this takes place, the metals being applied are no longer “available” to the biomass.  When this takes place, inorganic salts are often blended with chelates to ensure a portion of the micronutrient (metal) remains soluble.   Typically, the chelates being applied for this need are EDTA, citric acid, or gluconic acid,

For those looking to leverage the modern advances within technical water treatment, proprietary and scientifically balanced multi-metal blends are available.  When used, these additives can noticeably reduce a plant chemical footprint as the mix of nutrients within are engineered to match the stoichiometric consumption ratios that are present in healthy biomass.

Calculation/Identification of Micronutrient Deficits and their Correction

To restore or maintain proper micronutrient levels within an activated sludge system, the following must be considered; Influent water flow, baseline nutrient concentrations already present in the bulk water and the residual concentrations needed to ensure optimal performance.

To calculate how much of a specific micronutrient is missing, a fundamental engineering calculation must be applied.  That calculation is defined as the “Deficit Equation” which is listed below.

Mass Deficit _{(Target Nutrient)} = Q x (C _Target – C _Actual) x 1/1000

Where:

Mass Deficit _{Target Nutrient} = Mass Deficit of the Micronutrient being evaluated (kg/day)

Q = Flow Rate of the Influent Wastewater (m³/day)

C _Target = Desired micronutrient concentration in the bulk water of the aeration basin (mg/L)

C _Actual = Concentration of micronutrient already present in the bulk water of the aeration basin (mg/L)

1/1000 = is the conversion factor for the units (mg/L is equivalent to g/m³)

Because micronutrients are added as chemical compounds rather than pure elements, the mass fraction (MF) of that micronutrient within the chemical being applied must be accounted for.  In this case, the resultant “Product Dosing” calculation would be.

Product Feed Rate (kg/day) = Mass Deficit / Mass Fraction of active in the Chemical Additive.    

Sample Calculation

To ensure comprehension of these calculations, an example focusing on iron (Fe) has been provided.

An industrial food processing facility has a wastewater treatment plant in place and operates its activated sludge system under the following conditions:

• Influent Flow Rate (Q): 5,000 m³/day
• Target Iron Concentration: 1.0 mg/L
• Micronutrient Additive Used (Fe) – Liquid Ferrous Sulphate (FeSO₄)
  • Iron Content – 10% (mass)
  • Specific gravity – 1.25 kg/L

Within the secondary treatment portion of the wastewater treatment plant, chronic filamentous bulking is being observed.  An ICP analysis of the bulk water of the aeration basin has revealed that soluble iron is low and measured at 0.12 mg/L.  To restore performance, the iron residual must be increased re-stablish its target concentration of 1.0 mg/L.

Step 1: Calculate the daily mass of pure iron needed to close the gap between 1.0 mg/L (Target and 0.12 mg/L (current residual).

Mass Deficit (Fe) = 5000 m3/day x (1.0 mg/L – 0.12 mg/L) x 1/1000 = 4.4 kg/day (Fe)

As the plant uses commercial grade ferrous sulphate, the technical team must consider the amount of iron that is present in the ferrous sulphate solution and how much “product” will be needed to meet the 4.4 Kg/day deficit that was identified.

Product Feed Rate (kg/day) = 4.4 Kg/day / (10/100) = 44 Kg/day of FeSO₄ as product.

On a volume basis, 44 Kg/day equates to 44 Kg/day/1.25Kg/L =35.2 L/day of FeSO₄ .

By establishing a ferrous sulphate injection of 35.2 L/day, iron residuals will quickly recover, and filamentous bulking will start to decline.

As one can surmise, with a simple shift from reactive troubleshooting to proactive control, many operational performance bottlenecks can be resolved.  Whether it be an enhancement in contaminant removal kinetics or development of a more resilient floc, taking the time to routinely test and correct for micronutrient deficiencies will pay dividends in the long run.

Need help getting started?  Aquasan’s team of experts are close by and can provide the support you need to both assess your “status quo” and apply chemistries if a deficit has been identified.

At Aquasan, your success is how we measure ours!