Granules were removed from reactor D and kept under reactor liquor and refrigerated until processing was commenced. Processing was as follows: The granules were fractured fresh from the reactor liquor to provide the surfaces and particle size desired
for the scanning electron microscope (SEM) work. The primary fix was 3 %
glutaraldehyde, 2 % formaldehyde i n O. I M phosphate buffer at pH 7 . 2 !eft overnight.
Samples were washed three times in the same buffer as above. An acetone
dehydration series consisting of 25%, 50%, 75% and 95% v/v acetone in water for
1 5 to 20 minutes each and 2 x 1 00% for 90 minutes each was used. Samples were
_ t!'ten critical point dried using liquid carbon dioxide. The dried samples were glued
using conductive silver paint to an aluminium SEM stub and sputter coated with gold.
3. 13
microscope ·was used to study and photograph the samples. The protocol was recommended and carried out by the staff of the Electron Microscope Unit of the Batchelar Research Centre (Hort Research, Palmerston North, New Zealand). Some of the funding for this unit was provided by the New Zealand Lottery Grants Board. 3.5 CONTINUO US DIGESTION EXPERIMENTS
3.5. 1 Reactor B . An upflow anaerobic sludge blanket reactor
This reactor was constructed from "QVF" glassware (Quickfit Visible Flow process plant and pipeline, Coming Process Systems, Coming Ltd, Stone, Staffordshire, England). The apparatus is illustrated in Figure 3.2. The working volume, including sidearm and tubing was 20.4 e. No gas solids separator was installed because it was thought that a high degree of solids retention would increase the wood /micro organism ratio in the bed to a detrimental level. The expanded diameter at the top of the reactor allowed a decrease in superficial velocity to aid solids settling. Rising gas bubbles from the column generated a liquor circulation in this chamber such that the flow at the walls and overflow point was
�
ownwards. This enhanced solids separation to some extent. It was expected lhat this design would provide sufficient retention of methanogenic biomass for effective reactor operation. Initial reactor operation confirmed that the biomass retention was sufficient. Reactor liquor overflowed into a side arm from which the reactor recycle was withdrawn. A solids wasting point was provided at the bottom of the side arm. The reactor effluent waswithdrawn from the side arm by overflowing through an inverted siphon to provide a gas seal.
Gas was removed from the top of the reactor through thick walled butyl rubber tubing to a condensate trap and a wet work experimental gas meter (Brand catalogue number 2505.62, Rudolf Brand GMBH + CO, Wertheim-Glashiite, West Germany). Bubbling the gas through the condensate trap gave a visual confirmation of gas evolution and provided a small positive pressure in the reactor.
Samples for pH and volatile fatty acid analysis were removed via a junction in the recycle line. This was to avoid changes in pH due to carbon dioxide evolution and changes in volatile fatty acids with time in the effluent bucket. Feed and effluent samples were taken as representative samples of the mixed feed or accumulated effluent. These were usually 24 hour samples. Sludge samples were taken by insertion of a semi-rigid tube through the top of the reactor.
Reactor B was installed in a constant temperature room at 35 ± 2 °C. Any particular point in the room varied by ± 1 °C but there was some temperature stratification. Thus the air temperature next to the reactor from bottom to top increased by about
2 °C. The condition of the cork and plaster ceiling prevented the use of a ceiling fan
to give a more uniform room temperature. Smaller fans about the room did reduce the amount of stratification. Recycling of the reactor liquor ameliorated the temperature range experienced within the reactor.
Some evidence that the sludge bed of reactor B was poorly mixed was observed. A triangular wire frame stirrer as shown in place in Figure 3.2 was installed. This was driven at one r.p.m. by a Masterflex fixed speed pump drive ( 1 r.p.m., model 7544-01) for 15 minutes every 2 hours. The stirring action resulted in increased
.
reactor inlet blockages and no improvement in reactor performance was noted. Use of the stirrer was discontinued but it was left in place.
Figure 3.2 Reactor B (S) N \0 Feed SU rrer Gos Recyc l e Po int Recyc l e 3.15
3.5.1.1 Feed and recycle provision.
Feed and recycle were pumped to an inlet at the bottom of the reactor using Masterflex peristaltic pumps incorporating variable-speed drives (model 7546-00) fitted with interchangeable pump heads and silicone rubber tubing (Cole-Palmer International, Chicago, lllinois, U.S.A.). To reduce blockage problems pump tubing of 16 series (3. 1 mm nominal diameter) was used. To maintain the desired feed flow
rate the pump drive was controlled with a cam-timer to give a pump run time of
approximately 15 seconds every minute. The recycle pump used 15 series (4.8 mm
nominal diameter) tubing. Solids in the feed and effluent caused abrasion in the pump tubes, especially at the folding line, which shortened the tubing life markedly. Typically failure occurred after about three weeks. Replacement with Norprene (Reg. T.M. Norton Company, Masterflex catalogue number 6404-) eliminated this abrasion problem.
Flow tubing for the reactor was originally flexible PVC joined with 'Serkit Systems' fittings (Auckland Tool and Gauge Co. Ltd, Takapuna, Auckland, New Zealand) and hose clips. Unfortunately, even when drilled out, the constriction in flow diameter at the fittings caused blockages due to accumulation of resinous solids and mineral scale. A change to semi-rigid nylon tubing joined externally with offcuts of Masterflex silicone greatly extended the time between blockages.
Feed was from stirred glass or polyethylene vessels from within the controlled temperature room. This was done to provide the shortest possible feed line between feed vessel and feed pump. If these lines were too long settling of the wood fibres caused blockage problems in the pump tube. After an infection at the pulp mill from which feed was sourced, presumed to be Klebsiella species, volatile fatty acid production in the feed vessel became a problem. To reduce this the feed vessel was moved to a nearby cold room and the feed was circulated to the feed pump. The modified feed system is shown in Figure 3.3. The use of a Shacklock dishwasher pump (part number 388344) gave sufficient mixing on return to keep feed solids suspended.
211 "t: Room
Feed to reactors
Figure 3.3 Final feed system.
3.5.2 Reactor A: A modified upflow anaerobic sludge blanket reactor
3. 17
Reactor A was also constructed from "QVF" glassware with a working volume of
12 e. It was similar in concept to reactor B but had certain significant differences which are shown in Figure 3.4 as compared to Figure 3.2. The settling zone or
"head" of reactor A was considerably smaller than that of reactor B so that suspended solids settling was reduced compared to reactor B. No mechanical stirring was provided for reactor A.