Overview

As a general rule, maintenance of agitation, pumping and flushing equipment should always be performed above ground level [5-6]. If this is not possible, a forced ventilation system must be permanently installed in order to counteract the risk of asphyxiation and poisoning in the event of an escape of gas.

Consider these basic operation and maintenance principle:

To strive and perform optimal, “life support systems” must work at all times, and “living conditions” shall be kept stable and in harmony with process requirements^[1]
Proper biogas O&M is all about stability and harmony, well in line with old Chinese Confucianism principles.^[1]
General good practise O&M procedures can be defined, but also a “feeling” for the individual plants requirements is needed by the O&M staff. ^[1]

Design Interaction

Suitable and sturdy equipment
Automatic protection and safety instrumentation to avoid serious break downs
Redundant installation on key functions, or quickly replaceable complete components on stock
- Operation quickly restored
- Proper service off-line
Shut off possibilities and easy access where relevant
Drain and flushing connections where service is foreseen
Process indication instruments and logging to be able to follow equipment state and trends, in order to react in time

Spare Part Management

Keep strategic spare parts and wear parts on stock!
- Strategical spare parts can be foreseen to be needed to keep vital functions running and/or parts with long delivery time
- Wear parts are parts which can be foreseen to wear out periodically, to be able to schedule service without need to wait for parts
Update relevant spare and wear parts to have on stock according to experience
Remember to replenish stock as soon as spare parts are used
Consider to refurbish replaced equipment to keep as emergency spare units

Storage of spare parts:

Keep storage facility dry, clean and ventilated.
Keep spare parts organized so you know where to find the parts for different equipment.
Keep parts according to manufacturers recommendation
Upon receiving of parts: Check that it is the right parts and quantity. Put them in place.
Spare parts are normally preserved for shipment and storage when delivered, so keep in original packing

Comminutors

According to information supplied by the manufacturers, the equipment is of low-maintenance design. It should be possible to carry out maintenance during the breaks in feeding.^[2]

External Comminutors

Maintenance can be arranged by contract with the manufacturer and is a necessity, depending on the substrates
worked. A supply of comminuted material to bridge downtimes for maintenance can be stocked on site.

In-pipe Comminution Agitators

Freestanding units can be serviced quickly without long outages. Easily accessible apertures significantly speed up cleaning.

Characteristic values	Perforated-plate comminutors up to 600 m³/h delivery rate, motor power ratings between 1.1 and 15 kW Inline twin-shaft comminutors based on rotary displacement pumps: comminution rates up to 350 m3/h • Characteristic values depend heavily on dry matter content. Delivery rate drops off sharply as dry matter content increases.
Suitability	Perforated-plate comminutors suitable for substrates with fibre content Inline twin-shaft comminutors also suitable for pumpable substrates containing higher proportions of solids
Advantages & Disadvantages	+ Easy accessibility to the equipment in the event of a breakdown + The units are easily opened and serviced in the event of clogging + Interfering substances are stopped by built-in separator trap (perforated-plate comminutor) - Dry matter content in the digester can be increased only up to the limit of pumpability of the substrate - Substrates containing interfering substances can cause accelerated wear (inline twin-shaft comminutor)
Designs	Perforated-plate comminutor: rotary blades in front of strainer Inline twin-shaft comminutor: shafts fitted with cutting or ripping tools

Comminutors Combined with Conveyor Technology in Single Units

Freestanding pumps can be serviced quickly without long outages; submersible pumps are easily removed from the substrate for servicing. Maintenance apertures make for much shorter downtimes.According to information supplied by the manufacturers, the equipment is of low-maintenance design. Maintenance contracts are available.
It should be possible to carry out maintenance during the breaks in feeding

Hygienisation Tanks

The tank must have at least one manhole. Comply with applicable health and safety regulations for working inside enclosed spaces
(due consideration also has to be given to gas safety regulations) The equipment as such (temperature sensors, agitators, pumps) has to be serviced; the tank itself should be maintenance-free.

Pre-digester Pit

Depending on the type of agitator, maintenance can be undertaken outside the pre-digester pit or the digester, without process interruption.

If the design lacks provision for removing settlement layer material, this material has to be removed manually. Apart from this, virtually no maintenance outlay; maintenance of the various technical items of equipment is described in the corresponding sections.

Infeed of the Substrate

Pumps

► Pumps for Biogas Plants

Rotary pumps

More difficult in the case of submersible pumps, although relatively easy access through removal apertures. Comply with applicable health and safety regulations for working inside the digester. Outages tend to be slightly longer than for other types of pump.

Eccentric single-rotor screw pumps

Very durable!
The design is inherently service-friendly; outages are short on account of the quick-change design of the screw drive.

Hopper pumps for solids infeed into piped liquid

The design is inherently service-friendly; outages are short.

Rotary displacement pumps

The design is inherently service-friendly; outages are short.

Infeed Screw Conveyors

Moving parts, so outlay for regular servicing must be taken duly into account. Manual intervention required to clear clogging or remove trapped interfering substances. Servicing of the screw that feeds to the digester necessitates what can be considerable downtime.

Infeed with Ram Feeder

Moving parts, so outlay for regular servicing must be taken duly into account. Servicing of the ram feeder necessitates what can be considerable downtime, possibly also associated with emptying of the digester.

Digester

► Material for Biogas Digesters

Stirred-tank Biogas Reactors

Manhole facilitates accessibility.

Biogas Reactor with Plug-flow

At least one manhole is necessary so that the interior of the reactor can be accessed in the event of a breakdown.
Comply with applicable health and safety regulations for working inside the digester

Integrated Heating Systems

Heaters have to be cleaned regularly in order to ensure that heat transfer remains effective
Access to heaters integrated into the digester or the structure is very difficult, if not impossible
Comply with applicable health and safety regulations for working inside the digester

External Heat Exchangers

Very good accessibility for maintenance and cleaning

Machinery for Mixing in the Digester

Submerged-motor Propeller-type Agitators

In some cases difficult, because the motor has to be lifted out of the digester
Maintenance hatches and engine-extraction hatches have to be integrated into the digester
Comply with applicable health and safety regulations for working inside the digester

Long-shaft Agitators

Motor maintenance is straightforward on account of siting outside the digester; no need to interrupt the process
Repairs to propeller and shaft difficult, because they have to be taken out of the digester or the fill level inside the digester has to be lowered
Maintenance hatches have to be integrated into the digester
Comply with applicable health and safety regulations for working inside the digester

Axial Agitators

Motor maintenance is straightforward on account of siting outside the digester; no need to interrup in the process
Repairs to rotors and shaft difficult, because they have to be taken out of the digester or the fill level of the substrate in the digester has to be lowered
Maintenance hatches have to be integrated into the digester
Comply with applicable health and safety regulations for working inside the digester

Paddle/paddle-wheel Agitators

Motor maintenance is straightforward on account of siting outside the digester; no need to interrupt the process
Repair of paddles and shaft difficult, because the digester has to be emptied
Maintenance hatches have to be integrated into the digester
Comply with applicable health and safety regulations for working inside the digester

Pneumatic Mixing

Gas-compressor maintenance is straightforward on account of siting outside the digester; no need to interrupt the process
Repair of biogas injection equipment difficult, because the digester has to be emptied
Comply with applicable health and safety regulations for working inside the digester

Hydraulic Mixing

Equipment specific maintenance is similar to that of the pumps stated above.

Sediment Discharge and Removal Systems

Maintenance of permanently installed systems requires emptying the digester, so out-of-tank drives or removal equipment have advantages
Comply with applicable health and safety regulations for working inside the digester

Gas Storage Tanks

Maintenance-free to a very large extent.

Flare System