Wastewater treatment, a working reference.

Sludge is the solids fraction of wastewater. It comes in two flavours that combine into a single feed to the digesters:

• PrimarySettles out at the head of the plant, before biological treatment. Mostly organic solids, fats, undigested matter.
• SecondaryBiological solids produced during aerobic treatment. The biomass that grew while consuming dissolved organics.

Once the combined stream enters a digester, it's raw sludge. Once it comes out, it's digested sludge. Cellulose is a major component, and a substantial portion of municipal-wastewater cellulose comes from toilet paper.

Composition is broadly stable across municipalities. The influent in San José isn't materially different from Cleveland or Phoenix on a residential basis. The exception is industrial influent. Maddy gave Duluth, Minnesota as the example: ~50% of that plant's influent is from a single paper mill, which gives it a completely different character. San José is dominantly municipal, so the standard process design assumptions apply.

Anaerobic digestion is the biological breakdown of organic solids in the absence of oxygen, performed by communities of bacteria that consume organic matter and produce biogas (mostly methane and CO₂) as a metabolic by-product.

The dominant heat duty isn't maintaining digester temperature. It's bringing raw thickened sludge from inlet temperature up to digestion temperature. That's where most of the heat input goes. Once at temperature, the digester is reasonably well-insulated and only needs makeup heat for ambient losses and for new feed coming in.

This matters for MicroLink because the duty where heat is consumed — raw sludge preheat from 15–20 °C up to 35 °C — is exactly the duty that 45–55 °C waste heat is well-matched to. The high-grade cogen heat (80–85 °C) currently doing this work is over-specified for the duty.

MHP targets the hard-to-digest cellulosic material that conventional anaerobic digestion can't break down — the 40% that would otherwise leave as digested sludge. It uses a specialised bacterial culture engineered for cellulose hydrolysis.

Rather than running the entire digester at MHP conditions, a fraction of the digester contents is diverted, treated in the MHP reactor, then returned. This concentrates the cellulose-attack chemistry in a small dedicated vessel rather than imposing it on the full 3-million-gallon volume.

Less digested sludge means smaller dewatering equipment, less truck haulage, lower disposal costs, smaller footprint for the bio-solids handling train. The capex saving on those downstream systems can partially offset the capex of the MHP module itself.

First full-scale MHP facility is currently under construction in Denmark. Until that plant is commissioned, all MHP performance data comes from pilot-scale or technology-vendor projections. San José will be the largest active MHP deployment in the United States once built.

• Higher feed concentration. Pre-treated sludge is less viscous and easier to handle, allowing the digester to be fed at up to 10% solids instead of 4–6%. The same digester volume processes much more dry mass — effective intensification of existing infrastructure.
• Better digester kinetics. The pre-broken material is more accessible to the bacteria, so conversion is faster and more complete.

A large facility in Sacramento has a $140M, 15 MW biogas utilisation contract. Useful comparator for the scale of capital being deployed in this sector — and the kind of capex required for serious beneficial use.

The technical case for MicroLink at San José isn't about adding heat to a heat-starved plant. The plant already has more cogen heat than it needs for current operations. The case is about thermal substitution and capacity unlocking.

Same volume of biogas, producing more useful work. Same digester volume, processing more solids. Same site footprint, generating more output. The plant becomes more productive without becoming larger.