Method To Evaluate Pre-Treatment Technologies For Anaerobic Digestion Of Sludge

Stephenson, R., Skene, G., Drew, A., Britch, D., Laliberte, S., and Hoy, P.
Paradigm Environmental Technologies Inc., Canada


Sludge management occupies a large portion of most wastewater treatment plants’ capital and operating budgets. However,
the generally poor effectiveness of anaerobic digesters to convert sludge to biogas has encouraged the industry to consider
sludge pre-treatment to boost digester performance. Concerns of particular importance include increasing biogas production,
decreasing residuals for disposal, and increasing digester throughput in a limited footprint. Additionally, growing public
support for sustainable energy provides an environmental incentive for WWTPs to become renewable energy facilities, rather
than net loads on power utilities.

Evaluating different sludge pre-treatment technologies is complex for a number of reasons. While sludge pre-treatment
technologies share a common overall objective of increasing anaerobic digester performance, both the equipment and their
results can be vastly different, making it difficult to compare costs and benefits.
Both the practical and economic value of benefits due to different pre-treatment methods may also vary from plant to plant.
For example, sludge pre-treatment that results in faster digestion can provide increased digester capacity. The value of this
depends on whether or not a WWTP’s digesters are capacity constrained with insufficient land available or if the WWTP has
capacity to spare. No two operators of wastewater treatment plants will necessarily make the same choices for their facilities
due to different priorities.

A method is needed to take these many factors into account and assist owners and operators of wastewater treatment plants
in making prudent decisions about sludge digestion. The Sludge Pre-treatment Economic Benefits Calculator (the
“Calculator”) was developed to meet this need.

The Calculator is an Excel tool that can be used to estimate the annual operating and maintenance costs or savings, the
equivalent annual value (EAV), and the net present value (NPV) for any sludge pre-treatment equipment compared to
anaerobic digestion without sludge pre-treatment. It can therefore be used to compare the economic benefits for different
sludge pre-treatment technologies.

This paper is both the User Guide for the Calculator and the Calculator itself. It explains the terms, the assumptions, and the
calculations used. It has six sections, as follows:
1. User defined inputs for sludge characteristics and quantities, anaerobic digestion performance, and input operating
costs for any sludge pre-treatment technology and for any anaerobic digester baseline performance level;
2. Operating cost outputs that are summarized as residuals disposal cost, value of energy recovery, and sludge pretreatment processing cost;
3. The difference between operating benefits and costs with a sludge pre-treatment technology and a base case;
4. User defined inputs for capital costs items such as sludge pre-treatment equipment, anaerobic digesters, land, residuals
dewatering equipment, sludge storage, and biogas to energy conversion equipment;
5. The absolute and difference in capital costs with a sludge pre-treatment technology and a base case; and
6. An estimate of Net Present Value (NPV) savings and Equivalent Annual Value (EAV) for any sludge pre-treatment
technology compared to a base case.

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