Currently, mechanical dewatering technologies can remove 15 % to 30 % of the initial moisture content. Some technologies (e.g. filter presses) can remove water contents of up to 35 % to 50 %, but require significantly higher time, energy and chemical additives for dewatering.
In order to achieve dry matter contents above 50 %, FOW and sludge are mainly dried with two existing approaches: Thermal and solar drying. Although existing FOW and sludge drying process were mainly derived from standard technologies, however a direct adaptation of existing drying processes is difficult due to odor generation and stickiness of drying sludge
Thermal dryers commonly use fossil fuels, biogas or biomass for convection and conduction dryers. Several techniques are available for thermal FOW and sludge drying like rotary dryers, fluidized bed dryers, belt dryers and flash dryers. However, thermal drying requires high investments and shows a significant energy demand resulting in high operational expenses.
As an alternative drying method with lower operational costs, solar drying of FOW and sludge is proposed using solar radiation as energy source for evaporation. The process of solar drying is described in detailed in the relevant literature and shows high potentials in replacing thermal mechanical dryers for tropical countries like India. The overall suitability of solar drying of sewage sludge in India has been already investigated and demonstrated through a pilot plant at Mysore Campus of Infosys Ltd., Bangalore by the project partner ISAH, Hannover.
Solar Updraft Dryer – 3D Design (Work in Progress)
Dewatered sludge shows a tendency to foam during mixing and turning. For simple applications, dry materials like sawdust are added to the sludge to increase the dry matter content. Other fibrous materials like redundant FOW from vegetable markets (e.g. banana peduncle, banana stem) with similar characteristics as sawdust are assumed to be a potential additive for improving the drying characteristics of sludge, as banana peduncles and stems in India are characterized by a high share of cellulose and lignin. Although usually no additional thermal energy is required for the drying process and hence operational expenses are lower than for thermal drying, available solar greenhouse drying concepts show an intermittent or continuous power consumption for forced air exchange by blowers. Solar dryers using natural convection for exchanging drying air are available for agricultural applications in tropical and subtropical countries. They can be characterized by low mechanical complexity and require less investments and maintenance than solar dryers with forced ventilation. Through solar radiation the internal temperature of natural convection solar dryers increases above the ambient temperature. The associated expansion of the moist air inside induces a vertical airflow based on buoyancy, leaving a slightly negative pressure within the solar dryer, which again stimulate the inflow of ambient air. Solar drying concepts using natural convection have been demonstrated for food products. However, natural convection impedes any control over the airflow rate. Airflow in natural convection solar dryers for agricultural applications ceases completely during night time and adverse weather conditions, which can further lead to moisture condensation within the dryer and subsequent rewetting of the substrate.
By contrast, solar induced ventilation has been studied during the last two decades to ensure a natural and sustainable air exchange in buildings of subtropical and tropical countries. In particular, the concept of solar chimney ventilation shows a high suitability to substitute mechanical building ventilation and reduce operational expenses, energy demand and greenhouse gas emissions. The solar chimney effect has also been analyzed in detail for power generation in a solar chimney power plant, including plant dimensions and mathematical process modelling. Comprehensive mathematical models for designing a solar chimney facility exists. This has proven the practical feasibility of solar chimney power plants with a pilot plant in Manzanares, Spain. They observed an economic power generation above a chimney height of 700 m to obtain a higher airflow in the turbines, which however would not be required for a simple drying operation. A chimney effect can be maintained even during nighttime by using water bags or ponds under the transparent solar collector surface, which save solar energy during the day and release it during night. The temperature increase within the solar collector is dependent on transmission coefficient of the transparent collector cover and the radiation absorptivity of the collector’s floor material. Darker materials show a higher radiation absorptivity.