A Model for Digestion Enhancement

Le, M.S. and Hu, R., United Utilities Group Plc



Historically, digestion was simply regarded as a mean for sludge stabilisation to avoid odour nuisance.  Over the years, as the available land bank for agricultural recycling became more limited so interest grew in better volatile solid destruction with digestion.  The introduction of the Safe Sludge Matrix at the end of the nineties shifted the emphasis to improved pathogen reduction.   In more recent years, however, digestion has received a new lease of life, as a process for renewable energy production.  The fundamental biological process has remained the same over years; what has sprung up along side the basic process is a plethora of pre-treatment techniques that have been developed to enhance one or more aspects of the process performance.

Many of pre-treatment technologies are able to provide significant benefits, such as higher biogas production as well as lower sludge disposal cost.  It also appears that certain techniques suit certain situation better.   However, there is currently no model that would enable a user to make an objective evaluation of a technology for a particular situation and more importantly for the prediction of an optimal solution in a future scenario where the political and economic environment would be totally different from today.

This paper provides a detailed review of the pre-treatment technologies.  It describes a universal model that takes into account the price of energy, the cost of agricultural recycling, and the cost of capital in order to predict the best solution.  Sensitivity analysis shows VS reduction, de-waterability and energy consumption to be the most significant factors in digestion pre-treatment. The model also provides a useful pointer for the development of the next generation of highly efficient digestion enhancement technologies.

Keywords: Digestion enhancement; sludge; volatile solid destruction; biogas production


Anaerobic digestion

There are more than 36,000 anaerobic digesters (AD) today in operation in Europe, treating around 40 – 50% of the sludge generated from wastewater treatment (Mata-Alvarez et al., 2000).  United Utilities (UU), for example treats around 75% of its sludge by the AD process with the aim is to achieve over 90% treatment in the next 5 years.

Mesophilic anaerobic digestion (MAD) is a treatment, which offers two major advantages for the water industry: firstly reducing the sludge quantity (half of the volatile solids is converted to biogas); and secondly stabilising sludge (by destroying pathogens and odour potential). Standard anaerobic digestion is normally a two-stage process. Primary digestion, where sludge is treated at approximately 35°C for 14-18 days followed by the secondary digestion where sludge is held for another two weeks in storage. The limitations with the current process are low VS reduction and methane emission from secondary digesters (as most of them are open tanks).

Enhanced digestion (Advanced digestion)

In anaerobic digestion hydrolysis, acidogenesis and methanogenesis take place consecutively. Where the process involves a solid substrate, such as sewage sludge, hydrolysis is often the rate-limiting step. Recently, there have been many attempts to improve sludge digestion with various pre-treatment techniques to achieve advanced digestion or enhanced digestion. Although entirely different in their mode of operation, all the techniques ultimately achieve improvement through an accelerated rate of hydrolysis.

The difference between standard digestion and advanced digestion can be summarised in Table 1. The key advantage by doing advanced digestion is to achieve higher VS reduction. Some of the technologies also offer better sludge dewaterability and sludge quality for agriculture recycling. Moreover, improving digestion can also reduce carbon footprint by either increasing the production of renewable energy or the elimination of the greenhouse gas emission from secondary digesters.   In some cases both benefits may be achieved simultaneously.

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