Co-Digestion of Organic Waste with Wastewater Solids Plant-wide Operational Impacts

Jay Surti, PE Residuals Practice Lead - Northeast

Sustainable Management of Food Waste

Co-Digestion Benefits for WRRFs

• The Synergistic Effect! Enhanced volatile solids reduction and biogas production. Enhanced biogas production can help offset operational costs or can serve as a revenue source through bio-CNG/LNG production

• Increase utilization of existing assets • Improve stabilized biosolids quality • Serve as environmental stewards for the local geography • Tipping fees can become an attractive revenue source • A new business opportunity for WRRFs!

However, it is Critical to Understand the Plant-wide Operational Impacts

Co-digestion is Emerging Rapidly National Co-digestion locations

The Water Research Foundation is Leading the Research

What is Organic Waste

Pre-processing to Produce an Engineered Slurry

The Anaerobic Digestion Process

Co-Digestion Effect on Digester pH and Alkalinity

• Stable digester operation: 6.5 to 7.6 • Carbohydrate based and fatty wastes

• Tend to decrease pH, alkalinity and ammonia concentrations

• Proteinaceous wastes • Tend to increase pH, alkalinity and ammonia concentrations

High C/N Ratio

Ammonia accumulation and helps with

buffering

Ammonia toxicity

Reduced buffering ability

Lack of N for biomass growth

Manure, animal wastes and other protein rich waste

Confectionary or

carbohydrate rich wastes

Combining the two type of wastes can

balance each other effects

Low C/N Ratio

Co-digestion Effect on Sludge Rheology

Viscosity vs. shear rate with and without HSW addition

Effect of Stoppage of Mixing on Gas Production Rates and Changes in Solids

Volume of a Digester

Source: Understanding Impacts of Co-Digestion, WERF, 2017

Unintended Consequences

Shock Loading

coupled with unoptimized

Mixing

Co-Digestion Impacts Flocculation and Dewatering

Co-Digestate Parameters: • Nature of HSW organics • pH and alkalinity • Conductivity • Hardness • Cations

Floc properties impacted:

• Size • Shape

• Density • Strength

Macro impacts:

• Flocculation • Settling • Dewatering • Membrane fouling

100µm

Monovalent to Divalent Cation Ratio Impacts Dewatering

Na+ + K+ + NH4+

Ca2+ + Mg2+

Divalent cations (Ca+2, Mg+2) and trivalent cations (Al+3, Fe+3) Fibrous feedstock e.g. rice straw

Monovalent cations (Na+, K+)

Monovalent Cations

Divalent Cations

Monovalent/Divalent Cations Ratio

Impact of Co-Digestion on Percent Cake Solids and Polymer Demand Source: Understanding Impacts of Co-Digestion, WERF, 2017

Inerts In Co-Digestate can Impact Biosolids Quality

Heavier fractions prior to pre-processing of pre- and post-consumer food waste • Stones, sand, glass • Metals, bones, silverware • Large Plastics and Wood

Lighter fractions after pre-processing of pre- and post consumer food waste • Microplastics • Grit, small stones • Sand, pieces of glass

Impact of Co-Digestion on Dewatered Cake Odors

Source: Understanding Impacts of Co-Digestion, WERF, 2017

Nutrients (N and P) in Organic Waste Source: Co-Digestion of Organic Waste Addressing Operational Side-Effects, WERF 2017

Nutrient Impact of Organic Waste Co-Digestion Source: Co-Digestion of Organic Waste Addressing Operational Side-Effects, WERF 2017

Nutrient Side Stream Treatment and Recovery

Side stream ammonia removal to minimize mainstream impacts

Struvite precipitation and P recovery

O&M Parameters and Indicators to Monitor for Co-Digestion

Digester Performance pH and Alkalinity in the digester Ammonia and VFAs in the digester Methane content of biogas COD of digester feed, digestate and recycle streams COD:VS ratio of HSWs Nutrients N:COD and P:COD ratios Cations M:D Ratios Inerts Ash content

Source: Co-Digestion of Organic Waste Addressing Operational Side-Effects, WERF 2017

System Design Considerations

Receiving, Screening, Homogenization, Odor Control

Mixing, Gas Handling and Materials of Construction

Economic Balance is Important to Co-digestion

Question and Answers

Jay Surti, PE Senior Associate and Residuals Practice Lead - Northeast Hazen and Sawyer 732-491-2817 (direct) | 973-953-9888 (cell) jsurti@hazenandsawyer.com