Gasification of Municipal Wastewater Primary Sieved Solids in a Rotary Drum Reactor

Municipal sludge contains significant amounts of energy, which can be exploited by the use of anaerobic digestion, combustion or gasification. However, the relatively high moisture content of dewatered biosolids (usually over 80%) and long field experience makes anaerobic digestion the most favourable process for energy production from biosolids. Micro-sieves are novel apparatuses for the removal of solids form raw wastewater through a sieving process, using a rotating fabric belt assembly with continuous solids scraping (Koliopoulos and Gikas, 2013). The removed primary sieved solids (PSS) are distinctively different from conventional biosolids, as they do not contain cellular mass. They also differ from primary sludge, as they are collected almost “intact” on the fabric belt, which allows for efficient dewatering through an auger device, producing biosolids with solids content above 40%. Biosolids with relatively high solids content are more suitable to undergo thermal processing (combustion or gasification) for treatment and exploitation of its energy content. Gasification is a thermal process for the reformation of organic substances, with carbon monoxide and hydrogen, as main products. Carbon dioxide and methane may also be formed in considerable quantities, while trace amounts of hydrocarbons may also be produced. The inorganic fraction of the feed is collected as ash (solid restitute). The gaseous product of gasification process is known as synthesis gas (syngas). Gasification is a process known since the 17th century (Miller, 2004), however, biosolids gasification has been exploited only recently (Fytili and Zabaniotou). Recent studies have shown that the electric energy produced by the gasification of conventional biosolids (at 80% moisture content) can be up to about two times the electric energy produced by anaerobic digestion (Gikas, 2014). The present study investigates the potential for the production of syngas from gasification of primary sieved solids from municipal wastewater, in an Ultra High Temperature (UHT) rotary drum gasifier. PSS (Table 1) had been collected from the wastewater treatment plant of Adelanto, Ca, USA, using an MS-54, M2Renewables, Inc., Ca, USA, microscreen (Fig. 1-left), and where shipped in two batches to the Pyromex AG facility in Munich, Germany, where gasification process took place. The microsieve was equipped with a cloth filter with openings of 350μm, however, the effective pore size was significantly lower, as the solids accumulated on the cloth filter act as a fine filter. The elemental analysis of PSS (ash free, dry product) is expressed by the following empirical formula: C5.0H8.1O3.3N0.10S0.01, while its high heating value (HHV) was measured as 19 MJ/kg (dry basis). The initial moisture content of the PSS was about 59%, while they were further air-dried to reach moisture content of about 17%.