Earth Systems Energy » Biosolids Processing

By: Earth Systems Energy  09-12-2011
Keywords: solid waste, Heavy Metal, Sewage Treatment

Hydrothermal Processing (HTP) – An Opportunity for Biofuel Production from Wet Biosolids Wastes


Over the past year Earth Systems has been actively investigating the use of high temperature pyrolysis (~700ºC) in order to increase eco-efficiency, reduce carbon footprints, and value-add to solid waste streams.

Large volumes of biosolids are generated at sewage treatment plants and through both adsorption and absorption can contain complex mixtures of chemicals as well as pathogens and viruses. This can present a disposal problem – their usage as a furnace fuel being a particular problem on account of heavy metal content. These stockpiles can also consume a large amount of space especially if they are dispersed for sun drying.

HTU presents a solution to these problems with a range of beneficial outcomes:

  • Generation of a crude bio-oil from which biodiesel could be produced;
  • Production of fuel gas to assist in driving the pyrolysis process and/or running other facilities;
  • The production of stable biochar for carbon sequestration and/or soil improvement;
  • Physical compaction of its on-site stockpiles; and
  • Pyrolysis of biosolids represents a tangible means for a water authority to offset its carbon budget.

What is the HTP Pyrolysis Process?

Hydrothermal Processing (HTP) is a thermo-chemical process for converting wet biomass feedstocks to crude biofuel products.   Feedstock moisture content is typically high (50% – 80% moisture is common).  The HTP process involves moderate temperatures and pressures (T = 300 to 370 ºC, P = 120 to 190 atm) and residence times of 5 to 20 minutes.  Water is present as a liquid under these conditions.  In simple terms, HTP can be described as a de-oxygenating process – removing oxygen from the biomass materials as water and CO2. The products from HTU are crude bio-oils (suitable for upgrading to biodiesel), gases (predominantly CO2 with some CO), water, biochar and ash.  The aid of a suitable catalyst may enhance methane (CH4) yield in the gas stream. Yields depend on the exact processing conditions, residence times, feedstock characteristics and catalytic effects.  The thermal efficiency of the HTU process is high (70 to 90%).

The bio-crude produced is not miscible with water and can be separated into light and heavy crude by extraction.  The light crude can be upgraded to a diesel substitute whereas the heavy crude can be co-fired with solid fuels (e.g. for power generation). Importantly, the process dramatically reduces the overall mass of biosolids and stabilises it to prevent ongoing GHG emissions.

The process is ideally suited to wet biomass feedstocks and organic wastes and essentially upgrades these wastes to second-generation biofuels.

Although the majority of activity in HTU development is pre-commercial R&D, there are significant moves afoot with demonstration / commercial-scale process development, the key economic outputs being gas and liquid fuels.

Application to Biosolids

There has been little reported in terms of HTP application to biosolids treatment, though the problem of sludge disposal is of increasing concern in many places (both in Australia and overseas).

Key issues to be considered when investigating the possibility of converting biosolids to biofuels via HTP-based technology are:

  • Sludge inorganics (ash) content, including heavy-metals – behaviour in the process and where/how these inorganics report in the final product streams;
  • Nitrogen content – What form does nitrogen take in the final products;
  • Bio-oil characterisation – is the bio-oil produced from sludge similar to that produced from other feedstocks – i.e. does it form an acceptable feedstock for upgrading to liquid fuels; and
  • Economics – is there sufficient economic merit in the use of sewage sludge as a feedstock for a bio-oil production process.

Current Work

Earth Systems is currently undertaking a scoping study to:

  • Evaluate the characteristics of particular biosolid feedstocks (originating in Australia and overseas), their behaviour in the HTP environment, and the relevant characteristics of the products generated (in particular, the bio-oil component);
  • Perform a feasibility study with Melbourne Water in using their biosolids for beneficial reuse, in particular for energy production (especially gas with high methane content) that is suitable for their current and future biosolids production and is able to accept feedstock with toxic metals.
  • Carry out a basic financial analysis considering the value of the biosolid disposal service and the value of the biofuel product.  This includes an estimation of the rough capital and operating costs of a HTP plant as a first assessment of financial viability for such a process.

Should the scoping study yield positive results, Earth Systems believes that the benefits of a HTP process could be applied to a number of Australian sites with biosolid waste streams. We view as highly advantageous an extension of this current work to involve additional local parties.

Keywords: Carbon Sequestration, Furnace Fuel, Heavy Metal, Organic Wastes, Reduce Carbon, Removing Oxygen, Sewage Treatment, Sewage Treatment Plants, Solid Fuels, solid waste, Treatment Plants, Waste Streams,

Contact Earth Systems Energy


Print this page

Other products and services from Earth Systems Energy


Earth Systems Energy » Biochar

These include a transportable batch process for processing materials in remote areas (i.e. too far away to be brought to a central processing plant), and a high-throughput continuous process suited to a broad range of woody materials designed for integration into existing waste management practices.


Earth Systems Energy » Bioenergy and Waste-to-Energy

Earth Systems has in-house engineering design expertise in thermo-chemical processes, so we have the capacity to develop a bioenergy or waste-to-energy solution specific to your feedstock and circumstances. With experience in sustainable biomass production, bioenergy plant design and carbon credit project development, we have the capacity to assist you in all aspects of developing your bioenergy project.


Earth Systems Energy » GHG Auditing

Earth Systems’ experience in the greenhouse gas auditing sector will ensure that your company’s emissions are properly managed through a site assessment to calculate these emissions and a provision of key recommendations to target the most cost effective methods of reducing these emissions.


Earth Systems Energy » Services

Earth Systems has experience in conducting studies to evaluate the most appropriate technology, energy requirements, social needs and sustainable aspects of an electrification project. Earth Systems has the expertise to identify energy efficiency improvements that could reduce your organisation’s energy costs while cutting your overall greenhouse gas emissions.


Earth Systems Energy » Small-scale Rural Electrification

Earth Systems understands that the specific social and environmental context for a project will not only dictate the technology choice but will also determine the key factors influencing the ultimate long-term success of the project.


Earth Systems Energy » Energy Efficiency and Auditing

As such, reducing energy consumption could not only improve your organisation’s financial performance, but would also reinforce your organisation’s corporate social responsibility in the environmental sector. Based on our experience in the energy sector, we will be able to give advice on how to use energy more effectively through key energy efficiency measures customised to fit your organisation’s environment.