DeNite Filter - Denitrification Filter

Post-denitrification process combining biological nitrate removal with solids filtration in a single deep-bed granular media filter.

Overview

The Denitrification Filter (DeNite) is a post-denitrification biofilm process using a submerged deep-bed filter with granular media. It provides tertiary treatment by removing nitrate through attached-growth denitrification while simultaneously filtering suspended solids.

When to Use

• Tertiary nitrogen removal after nitrification
• Plants requiring ❤️ mg/L total nitrogen
• Space-constrained sites needing compact tertiary treatment
• Retrofits to existing nitrifying plants
• Stringent nutrient discharge limits

Key Characteristics

• Combined denitrification and filtration
• External carbon source required (post-aerobic)
• High-quality effluent (low TSS and TN)
• Periodic backwash requirement
• Predictable performance with proper carbon dosing

Process Description

Operating Principle

The DeNite filter operates in downflow or upflow mode with wastewater passing through a bed of granular media (typically sand, anthracite, or specialized denitrification media). A biofilm develops on the media surface where denitrifying bacteria reduce nitrate to nitrogen gas using an external carbon source.

Nitrified     Carbon         DeNite          Low-N
Effluent  →   Dosing   →   Filter Bed   →   Effluent
(NO3-N)     (methanol)     (denitrification)  (N2 gas)

Carbon Sources

Carbon Source	Formula	COD:N Ratio	Advantages	Disadvantages
Methanol	CH3OH	3.0-3.5	Low cost, high efficiency	Flammable, acclimation needed
Ethanol	C2H5OH	3.5-4.0	Fast startup, safe	Higher cost
Acetic acid	CH3COOH	4.0-4.5	No acclimation, rapid	Corrosive, expensive
Glycerol	C3H8O3	4.5-5.0	Low cost, safe	Slower kinetics
MicroC	Proprietary	3.0-4.0	Optimized blend	Premium cost

Stoichiometry

Methanol-based denitrification: $$NO_3^- + 1.08CH_3OH + 0.24H_2CO_3 \rightarrow 0.056C_5H_7NO_2 + 0.47N_2 + 1.68H_2O + HCO_3^-$$

COD requirement: $$COD_{required} = 2.86 \times \Delta NO_3 + 1.71 \times \Delta NO_2 + 0.87 \times \Delta DO$$

Typical COD:N ratio with safety factor: 4.0-5.0

Biokinetics

Denitrification Rate

$$r_{NO3} = k_{max} \times \frac{S_{COD}}{K_{COD} + S_{COD}} \times \frac{S_{NO3}}{K_{NO3} + S_{NO3}} \times \frac{K_O}{K_O + S_O}$$

Where:

• r_NO3 = denitrification rate (g N/m3/h)
• k_max = maximum rate constant
• S_COD = carbon substrate concentration (mg/L)
• K_COD = carbon half-saturation (mg/L)
• S_NO3 = nitrate concentration (mg/L)
• K_NO3 = nitrate half-saturation (mg/L)
• K_O = oxygen inhibition constant (mg/L)

Temperature Correction

$$k(T) = k_{20} \times \theta^{(T-20)}$$

Parameter	theta Value
k_max	1.08-1.12
K values	1.0

Surface Area Loading

$$SNLR = \frac{Q \times \Delta NO_3}{A_{media}}$$

Units: g NO3-N/m2/d

Design Equations

Hydraulic Loading Rate

$$HLR = \frac{Q}{A_{filter}}$$

Design criteria:

Application	HLR (m/h)	HLR (gpm/ft2)
Low loading	2-4	0.8-1.6
Normal	4-8	1.6-3.3
High rate	8-12	3.3-4.9

Nitrate Loading Rate

$$NLR = \frac{Q \times NO_{3,in}}{V_{media}}$$

Design criteria:

Loading (kg N/m3/d)	Expected Removal
<0.5	>95%
0.5-1.0	85-95%
1.0-1.5	70-85%
>1.5	<70%

Empty Bed Contact Time (EBCT)

$$EBCT = \frac{V_{media}}{Q} = \frac{D_{bed}}{HLR}$$

Typical: 15-30 minutes

Carbon Dose

$$Dose = (COD:N) \times \Delta NO_3 + DO_{in} + 0.5 \times BOD_{in}$$

Where COD:N = 3.5-5.0 depending on carbon source.

Parameters

Filter Design Parameters

Parameter	Unit	Default	Range	Description
Bed depth	m	2.0	1.5-3.0	Media depth
Media type	-	sand	sand/anthracite/special	Filter media
Media size	mm	2-4	1-6	Effective size
Uniformity coeff	-	1.3	1.2-1.6	Size distribution
HLR	m/h	6	2-12	Hydraulic loading
EBCT	min	20	10-45	Contact time

Process Parameters

Parameter	Unit	Default	Range	Description
Influent NO3-N	mg/L	15	5-30	After nitrification
Target NO3-N	mg/L	3	1-8	Effluent target
Carbon source	-	methanol	various	Electron donor
COD:N ratio	-	4.0	3.0-5.5	Dosing ratio
Temperature	C	15	10-25	Operating temp

Kinetic Parameters

Parameter	Symbol	Unit	Default	Range	Description
Max denit rate	k_max	g N/m3/h	3.5	2-6	At 20C
Carbon half-sat	K_COD	mg/L	5	2-15	Monod constant
Nitrate half-sat	K_NO3	mg N/L	0.5	0.1-1.0	Monod constant
Oxygen inhibition	K_O	mg/L	0.2	0.1-0.5	DO inhibition
Temp coefficient	theta	-	1.10	1.08-1.12	Arrhenius

Backwash System

Backwash Triggers

• Headloss: >1.5-2.0 m
• Time: 24-48 hours maximum
• Effluent turbidity breakthrough

Backwash Sequence

Step	Duration	Air Rate	Water Rate	Purpose
Air scour	3-5 min	50-80 m/h	0	Break up biomass
Air + water	2-3 min	30-50 m/h	20-30 m/h	Combined cleaning
Water rinse	5-10 min	0	40-60 m/h	Final rinse
Settle	2-5 min	0	0	Media settling

Backwash Water Management

Backwash volume: 2-4% of throughput Return to: Plant headworks or dedicated treatment

Configuration

Example Configuration

{
  "type": "deniteFilter",
  "id": "tertiary-denit-1",
  "parameters": {
    "numberOfCells": 4,
    "cellArea": 50,
    "bedDepth": 2.0,
    "mediaType": "anthracite",
    "mediaSize": 3,
    "uniformityCoefficient": 1.35,
    "hydraulicLoading": 6,
    "ebct": 20,
    "targetNO3": 3,
    "carbonDosing": {
      "source": "methanol",
      "codToNRatio": 4.0,
      "injectionPoint": "upstream",
      "mixingDistance": 10
    },
    "backwash": {
      "airScourRate": 60,
      "waterRate": 50,
      "frequency": "headloss",
      "headlossTrigger": 1.8
    },
    "biokinetics": {
      "kMax": 3.5,
      "kCOD": 5,
      "kNO3": 0.5,
      "theta": 1.10
    }
  }
}

Worked Example

Given

• Flow: 20,000 m3/d
• Influent NO3-N: 18 mg/L
• Target effluent NO3-N: 3 mg/L
• Temperature: 12C
• Influent DO: 5 mg/L

Design

Step 1: Required Removal $$\Delta NO_3 = 18 - 3 = 15 \text{ mg/L}$$

Step 2: Temperature-Corrected Rate $$k_{12} = 3.5 \times 1.10^{(12-20)} = 3.5 \times 0.47 = 1.6 \text{ g N/m}^3\text{/h}$$

Step 3: Required Media Volume $$V_{media} = \frac{Q \times \Delta NO_3}{k \times 24} = \frac{20,000 \times 15}{1.6 \times 1000 \times 24} = 7.8 \text{ m}^3$$

Using NLR approach for safety: At NLR = 0.8 kg N/m3/d: $$V_{media} = \frac{20,000 \times 15 \times 10^{-3}}{0.8} = 375 \text{ m}^3$$

Step 4: Filter Area (2m bed depth) $$A_{filter} = 375 / 2 = 188 \text{ m}^2$$

Step 5: HLR Check $$HLR = 20,000 / (188 \times 24) = 4.4 \text{ m/h}$$ (within 4-8 range)

Step 6: EBCT Check $$EBCT = 375 \times 60 / (20,000/24) = 27 \text{ min}$$ (within 15-30 range)

Step 7: Number of Cells 4 cells at 47 m2 each (6.9m x 6.9m)

Step 8: Carbon Dose $$Methanol = 4.0 \times 15 + 5 = 65 \text{ mg/L as COD}$$ $$Methanol_{actual} = 65 / 1.5 = 43 \text{ mg/L}$$ Daily consumption: 20,000 x 43 / 1000 = 860 kg/d

Performance Monitoring

Key Indicators

Parameter	Target	Action Level
Effluent NO3-N	❤️ mg/L	>5 mg/L
Effluent TSS	<5 mg/L	>10 mg/L
Headloss	<1.5 m	>2.0 m
Carbon residual	<2 mg/L	>5 mg/L
Effluent DO	<0.5 mg/L	>1.0 mg/L

Troubleshooting

Issue	Cause	Solution
High effluent NO3	Insufficient carbon	Increase COD:N ratio
	Low temperature	Reduce loading
	Short-circuiting	Check distribution
High effluent COD	Carbon overdose	Reduce dose, add reaeration
	Poor backwash	Improve backwash
Rapid headloss	High TSS influent	Add upstream filtration
	Excess biomass	More frequent backwash
Poor denitrification	DO in influent	Add degassing or anoxic zone
	Wrong carbon source	Switch to acetic acid for startup
Gas binding	N2 accumulation	Increase backwash frequency

Integration

Connected Units

• Upstream: Nitrifying activated sludge, MBR, or trickling filter
• Downstream: UV disinfection, effluent storage, discharge
• Sidestreams: Backwash to headworks or separate treatment

Control Integration

• Flow-paced carbon dosing
• Headloss monitoring for backwash
• Effluent NO3 feedback trim

Related Topics

• MBBR - Moving Bed Biofilm Reactor
• Trickling-Filter - Trickling Filter
• Anoxic-Zone - Anoxic Zone
• CSTR - Continuously Stirred Tank Reactor
• Tertiary-Treatment

References

1. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8.
2. Tchobanoglous, G. et al. (2014). Wastewater Engineering, 5th Ed. McGraw-Hill.
3. EPA (2010). Nutrient Control Design Manual. EPA/600/R-10/100.
4. Stensel, H.D. et al. (2018). Principles of Biological Nitrogen Removal. IWA Publishing.