Feasibility of Thermal Utilization of Primary and Secondary Sludge from a Biological Wastewater Treatment Plant in Kaliningrad City

Abstract

Hydrothermal liquefaction (HTL) of sewage sludge is considered in the article as an analogue of the natural processes of oil formation (catagenesis). A comparison of the physicochemical composition of primary and secondary sludge with type II kerogen (natural precursor of oil) showed their similarity. Both types of sludge have a slightly higher level of oxygen and nitrogen. The study tested the hypothesis that the elements included in the inorganic part of the oil source rocks can have a catalytic effect on the oil formation processes. For the conducted studies of sludge HTL, the catalysts containing cations and substances found in oil source rocks were chosen: as homogeneous catalysts (KOH, NaOH, NH4Fe(SO4)2, CoCl6, NiSO4, CuSO4, ZnSO4, MoO3) and as heterogeneous catalysts (MgO, Zeolite, Al2O3). The effectiveness of catalysts containing metal ions, zeolite and aluminum oxide has been proven. The highest biocrude yield was achieved in a process with NiSO4 as the catalyst in a dose 2 g per 10 g of sludge: oil yield increased by 34.9% and 63.4% in the processing of primary and secondary sludge, respectively. The use of catalysts provided an increase in fuel HHV by 10.8–12.5%, which is associated with a decrease in oxygen content (by 10.8–43.2%) with a simultaneous increase in carbon (by 7.9–10.9%) and hydrogen (by 6.5–18.7%) content.

Effect of temperature on hydrothermal liquefaction of high lipids and carbohydrates content municipal primary sludge

“…In these conditions the HHV was 14.55 MJ/kg. This value is comparable with that obtained in other works: 10.55 MJ/kg [ 20 ] or 17.31 MJ/kg [ 28 ].…”

“…The authors declared that the four treatment plants have primary sludge with very different compositions and this, has conditioned the biocrude yield. In an older work, where the effect of catalysts on the HTL of primary sludge is compared, a yield of 30.7% based on volatile solids is reached, in the case of not using a catalyst [ 20 ]. Previously, in a continuous HTL process, a biocrude yield of 37.3% based on volatile solids has been reached [ 21 ].…”

Analysis and Comparison of Bio-Oils Obtained by Hydrothermal Liquefaction of Organic Waste

“…If we consider various approaches to processing these wastes, we will see that a number of papers [18,19] have been devoted to study of converting wastewater sludge via HTL, confirming the possibility of a fuel yield of 30% without use of catalysts [20]. Use of alkaline and metal-containing catalysts makes it possible to increase the fuel yield up to 50% [21,22]. According to numerous studies, bio-oil from wastewater treatment sludge has comparatively good quality, with a low content of nitrogen (3-7%) and oxygen (10-17%), and a higher heating value 22-48 MJ/kg.…”

“…The nature of the HTL process allows it to be used for raw materials with high moisture content and materials from a wide range of sources. Numerous studies have been published on the HTL of forestry and woody waste [4][5][6][7], agricultural waste [8][9][10][11], sewage sludge [1,[12][13][14][15][16][17][18][19][20][21][22], manure [23,24] and algae [3,16,25,26]. Choice of feedstock depends on a variety of factors, such as availability, cost of production and composition of the feedstock.…”

“…For example, the HTL of lignocellulosic materials leads to formation of a complex mixture of chemicals containing a significant proportion of oxygen [22]. After conversion of sewage sludge, the fuel contains a significant amount of nitrogen [21]. Algae contain carbohydrates, lipids and proteins [26][27][28] that break down into various organic chemicals, some of which contain nitrogen caused by deamination of amino acids from proteins.…”