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Food Waste Digestate Treatment

For Food Waste Digester Facility

Client Overview

  • Industry: Food Waste Digester Facility
  • Location: Michigan
  • Year: 2020
  • Facility Capacity: 100,000 GPD

Challenge

Our client, a Michigan-based Food Waste Digester facility, faced significant challenges with their digestate management:

1

Compliance with regulatory permits for digester run-off

2

High costs associated with land application of nutrient-poor runoff

3

Complex logistics for storage and hauling of leachate

4

Need for a solution to concentrate nutrients for efficient storage and later land application

Solution: Multi-Stage Filtration System

We implemented a three-stage filtration system to address these challenges:

  1. SWECO Vibratory Screen: For initial removal of large abrasive solids and fibers
  2. Ultrafiltration (UF): A unique polymer-free system for advanced filtration
  3. Two-Stage Reverse Osmosis™ (TSRO): For final purification of UF permeate

Project Approach

  1. Site Assessment:
    • Processed raw digestate through an 18″ diameter vibratory screen (447 microns, 37.9% open area)
    • Filtered screened digestate through our proprietary UF system
    • Further purified UF permeate using TSRO system
  2. System Specifications:
    • UF membrane average pore size: 0.02 μm
    • UF system: Stainless-steel modules with multiple porous tubular membranes

Results

1

UF Efficiency

Recovered 87% of the digestate as UF permeate
2

UF Performance

Achieved a high average flux rate of ~40-10 GFD
3

Water Reclamation

Recovered about 90% of the UF permeate as clean water through TSRO
4

Cost Reduction

Significantly reduced operational expenses (OPEX) for the client

Performance Analysis

 

Ultrafiltration (UF) Performance:

  • Total solids removal: Vast majority
  • Organic nitrogen removal: Significant
  • Phosphorus removal: Significant
  • COD reduction: 95%
  • BOD reduction: >78%

Analysis of Ultrafiltration Process Streams

PARAMETER Unit UF Feed UF Permeate UF Concentrate Percent Removed by UF
Total Solids mg/L 15,900 5,700 52,604 64%
Biochemical oxygen demand (BOD) mg/L 673 <149.2 NA >78%
Chemical oxygen demand (COD) mg/L 16,460 800 41,900 95%
Ammoniacal Nitrogen mg/L 1,020 971 1,210 5%
Organic Nitrogen mg/L 650 39 3,439 94%
Total Kjeldahl nitrogen (TKN) mg/L 1,670 1,010 4,661 40%
Phosphorus mg/L 281 12.2 1,500 96%
Potassium mg/L 1,010 953 1,300 6%
Sulfur mg/L 84 25.2 407 70%
Calcium mg/L 441 141 1,821 68%
Magnesium mg/L 75 52.6 203 30%
Sodium mg/L 1,130 1,120 1,318 1%
Iron mg/L 33.2 0.23 196 99%
Manganese mg/L 0.793 0.071 4 91%
Zinc mg/L 1.66 n.d. 9 100%
Copper mg/L 0.42 n.d. 2 100%
Conductivity ms/cm 14.1 13.9 NA n/a
pH S.U. 7.87 8.04 8 n/a

n/a: not applicable; n.d.: not detected , NA: not analyzed

As expected, the UF removed the vast majority of the total solids, organic nitrogen, and phosphorus from the digested food waste sample, producing a transparent permeate. The COD reduction was 95% and the BOD reduction was greater than 78%.

Two-Stage Reverse Osmosis™ (TSRO) Performance:

  • Produced very clean water with low levels of N, P, and K
  • Ammonia in RO permeate: 10.1 mg/L
  • Total nitrogen in RO permeate: 17.2 mg/L

Analysis of Two-Step Reverse Osmosis Process Streams

PARAMETER Unit TSRO Feed TSRO Permeate TSRO Concentrate Percent Removed by TSRO
Total Solids mg/L 5,700 200 28,100 96%
Chemical oxygen demand (COD) mg/L 800 22 5,400 97%
Ammoniacal Nitrogen mg/L 971 10.5 1,170 99%
Organic Nitrogen mg/L 39 0.2 60 99%
Total Kjeldahl nitrogen (TKN) mg/L 1,010 10.7 1,230 99%
Phosphorus mg/L 12.2 n.d. 9.69 100%
Potassium mg/L 953 2.7 1,650 100%
Sulfur mg/L 25.2 n.d. 74.4 100%
Calcium mg/L 141 n.d. 113 100%
Magnesium mg/L 52.6 n.d. 164 100%
Sodium mg/L 1,120 114 8,280 90%
Iron mg/L 0.23 n.d. 0.38 100%
Manganese mg/L 0.071 n.d. 0.157 100%
Zinc mg/L n.d. n.d. 0.05 100%
Copper mg/L n.d. n.d. 0.1 100%
Conductivity ms/cm 13.9 0.71 39.3 n/a
pH S.U. 8.04 5.71 8.27 n/a

n/a: not applicable; n.d.: not detected , NA: not analyzed

These data indicate that the TSRO produced very clean water with very low levels of N, P, and K. Only 10.1 mg/L of ammonia and 17.2 mg/L of total nitrogen was detected in the RO permeate. The high levels of sodium in the concentrate are an artifact of batch concentration on our pilot unit and is not expected commercially.

Key Benefits

  1. Regulatory Compliance: Produced clean water suitable for sewer discharge
  2. Cost Reduction: Lowered expenses for land application and transportation
  3. Nutrient Management: Concentrated nutrients for more efficient storage and application
  4. Operational Efficiency: Streamlined digestate management process
  5. Environmental Impact: Reduced risk of nutrient runoff into surrounding ecosystems
  6. Water Recovery: High recovery rates in both UF and TSRO stages
  7. Scalability: System design suitable for the facility’s 100,000 GPD capacity

Process Insights

  • The multi-stage system demonstrated exceptional performance in treating food waste digestate
  • The UF system’s ability to operate without polymers or processing aids significantly reduced operational costs
  • The TSRO system produced high-quality water suitable for various reuse applications
  • High sodium levels in the TSRO concentrate were noted as an artifact of batch concentration in the pilot unit and are not expected in commercial operation

Liquid Pictures

Scroll the gallery to see pictures of the materials from our filtration equipment.

  • Fremont_UF
  • Fremont_TSRO

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