Safety measures up storage of SEWAGE SLUDGE

With a growing need to store dried sewage sludge in bulk, a number of new risks are being presented to the sewage industry. Sludge that was previously disposed of at sea is now being dried and granulated for subsequent use as fuel or fertiliser. The dried sludge is a highly biodegradable material which can present a risk of explosion or spontaneous combustion.

As organic dust combusts, heat is produced. Moreover, oxidation of organic material produces an exothermic reaction. This heat rapidly increases the volume and pressure of the combustion products. The process is known as a deflagration, rather than an explosion, but the effects are no less damaging.

Spontaneous combustion

Dried sewage sludge is capable of self-heating and subsequent ignition, if the rate of heat production exceeds the rate of heat loss. A number of variables need to be considered when predicting the critical temperature, above which ignition can be expected. This figure is based upon the vessel proportions and test data of the sludge.

There is a further risk associated with self-heating, which is caused by a different mechanism. Excessive moisture, caused by condensation in the storage vessel, can cause fermentation or composting of the organic solids. This biological action can raise the temperature sufficiently to cause the exothermic reaction described previously.

It is important that the vessel proportions are considered with respect to the critical temperature. The hopper section geometry must be selected to ensure that the silo at least self-empties, thus preventing excessive storage times due to stagnant zones. The outlet size of the vessel must also be large enough to prevent bridging. The variable nature of the waste product means that these parameters are often underestimated.

Protection of the storage vessel against the damaging pressures associated with a dust explosion is usually best achieved by explosion relief venting, provided this can be discharged to a safe area. The silo can also be filled with an inert gas at all times to exclude oxygen. Inerting of the silo can be considered, but it requires high running costs.

The explosion can also be suppressed by detecting the incipient pressure rise and injecting the silo with a finely divided powder to `smother' the explosion. This protection is often best suited to smaller vessels, which are difficult to vent to a safe place.

A final option is containment, which in the case of dried sludge, requires the vessel and all connected plant to be designed for 8-9 barg.

Monitoring of the temperature in the storage vessels can be achieved with a multi-point temperature detector suspended from the vessel roof. This will monitor the temperature at the different layers, both during storage and silo filling.

Carbon monoxide

As the temperature in storage builds up, carbon monoxide gas is produced. Although production rates are low at first, and hence difficult to detect, Figure 1 shows the marked increase in carbon monoxide concentration as the temperature of the solids exceeds the critical temperature.

By continuous sampling of the silo atmosphere and temperature, the early onset of self-heating can be detected and alarmed.

In the event that self-heating has been detected early enough, the silo should be emptied. Flexible Big Bags have a limited temperature resistance, so outloading to bulk vehicles or skips is preferable.

If silo discharge cannot be carried out quickly, or an advanced exotherm was indicated from the temperature monitor, the silo should be flooded with an inert gas to prevent ignition. Sufficient gas should be provided to totally fill the empty silo. Carbon dioxide is typically used. This is heavier than air and is therefore best introduced through a plenum built into the silo outlet. Figure 2 shows the immediate effect on temperature and carbon monoxide production rates after activation of the inert gas system.

The scientific approach

The magnitude of the danger and damage associated with a runaway exotherm or dust explosion must not be underestimated. Loss of life is a distinct possibility and serious damage to the plant is inevitable.

A scientific approach to the system design, based on measurable parameters, can go some way to eliminating the hazards. The techniques for assessing and reducing the residual risks are now well established, but must be rigorously applied by suitably qualified and experienced engineers.

Ian Birkinshaw is with Portasilo, based in York.