Putting the heat on sludge
Simon-Hartley discusses the Eco-Therm sludge pasteurisation system
Eco-Therm Ohmic is a versatile, chemical-free, continuous flow technology that treats almost all types of biosolids. The heart of the process is an ohmic heater that delivers pathogen removal performance without the need for additional storage at high temperature.
The output from the heater passes through a heat recovery unit that is used to pre-heat the sludge feed to the ohmic heater. Raw sludge is screened before entry to a buffer tank from which progressive cavity pumps feed the heater through the heat exchanger. As raw sludge temperatures may vary by up to 20ºC, the small buffer tank is an essential component for the control of heater operation. Heater output is set at 80ºC and, with heat recovery designed to support a minimum feed temperature of 55ºC throughout the year, it is possible the temperature of the cool sludge may be higher than acceptable for digestion. It may therefore be necessary to install a cooler on the output from the heat exchanger to ensure digester temperatures can be accurately controlled.
Ohmic heating is a very efficient, direct contact, electrical process that has a typical thermal conversion efficiency of 95%. The system is widely used for pasteurisation in the food industry. An electric current is passed through the sludge and the conductivity of the fluid generates the heat required. Pathogen kill is achieved by thermal and electrical contact. The internal surfaces of the heater in contact with the sludge are formed from an insulating material. The electrodes are never hotter than the sludge being treated; this is an important factor in the elimination of fouling of the heating system.
The heater operating temperature has been based upon US Environmental Protection Agency (EPA) time temperature curves, which indicate that at 80ºC a maximum of only 72 seconds is required for complete pathogen removal. The heater assembly and a few feet of pipe downstream make any special provision for this retention unnecessary; in fact, testing has shown that the normal time to eliminate pathogens is so small that removal is achieved within the body of the heater.
The operating parameters selected reflect the feed to the ohmic heater, particularly with regard to conductivity, which varies with solids density, temperature and, to a lesser extent, sludge type. Conductivity increases with solids concentration and linearly as a function of temperature. The source of sludge has a lower impact on performance with primary sludges having a slightly better conductivity than activated sludge of the same concentration.
Eco-Therm represents the successful culmination of a complex series of investigations to isolate the variables that impact upon performance and operating costs. Each application is unique and many jobs include pre-thickening with a gravity belt filter. Process economics determine the feed concentration that will deliver the lowest life cycle costs, a calculation that reflects the trade-off between improved conductivity and the greater energy loss, from friction through the heat exchanger, as dry-solids levels rise.
The key element of any project is the end-disposal route and the conditions that apply to its use. Utilities generate sludge continuously and the quality standards for disposal sites are becoming ever more stringent, with quality related charges now mandatory in many countries.
United States' regulatory agencies have already acknowledged that Eco-Therm delivers biosolids compliant with the pathogen requirements for Class A, Subpart B, of 40 CFR Part 503 (EPA, 1992). The product is pH neutral and accepted as suitable for application to agriculture. In the Los Angeles basin Class A biosolids are now completely exempt from the land disposal charges that apply to sludges that have not been pasteurised. Farmers have showed a very strong preference for Class A biosolids that retain the traditionally high levels of nutrients and are now pathogen free. The lack of the burdensome management practices associated with the land disposal of less well treated products was also seen as a very positive reason to use pasteurised wastewater biosolids - something that may be equally applicable to the United Kingdom.
In the UK utilities have struggled to comply with the regulations for treated sludge by the end of 2001. Pasteurisation will be needed to meet the enhanced treated sludge obligations and the specific requirements for a 6 log minimum reduction and 1x10 E.coli limit for pathogenic organisms. In fact, the real cost difference between achieving 2 log or 6 log pathogen reduction is low and Eco-Therm remains competitive with conventional methods of achieving even small reductions in pathogen content. Further European legislation is expected and the need for pasteurisation seems firmly established. The most important aspect of Eco-Therm Ohmic is that process economics are less affected by size of installation than alternative systems and widespread use at small treatment plants is not prohibited by cost. The ease of operation means almost all plants have an adequate skills base. Training is straightforward and operational adjustments are simple.
The pasteurisation performance achieved by Ohmic heaters at 80ºC is second to none. A similar performance can be attained at lower temperatures but the minimum retention time increases dramatically. For example, in the US, the EPA 503 time-temperature algorithm dictates that sludge must be retained at 65ºC for at least 4 hours for a total pathogen kill whereas between 75ºC and 85ºC the retention time reduces from 6 minutes to 15 seconds. The choice of 80ºC enables any kill-time variations, which may result from normal control and power supply fluctuations, to be accommodated in the heater assembly. The downstream heat exchanger is independent of performance and can therefore be designed for peak heat transfer efficiency.
The control system ensures labour costs are low and, as no chemicals are involved, operating costs are mainly electricity charges. The residual heat in the feed to downstream digesters provides the total energy input for those units.
Dedicated heating equipment can thus be downgraded to standby status resulting in fuel and maintenance savings. The ability of the heater to pasteurise at a high dry solids content increases digester retention time and leads to a greater reduction in volatile material. The system improves the performance of downstream digesters, reducing their life cycle costs.
Eco-Therm is compact and complementary to existing processes, fitting easily
into most sites. The system can be deployed at both large and small STWs plants
and the fact that key components can be easily transferred from site to site
means that adoption of the process does not impinge on the strategic development
of utility services.