Putting the heat on sludge

The reliable removal of pathogens from sewage sludge has been identified as

a prerequisite for the safe disposal of biosolids. In a move thought to be unique

in process contracting, Simon-Hartley has introduced Eco-Therm Ohmic a pasteurisation

process using technology proven on sewage sludges for two years prior to launch.

This gives operators access to reliable operating data and the response of regulating

agencies to end disposal options.

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.

Compliance issues

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 1×10 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.