The stilling chamber describes the smaller compartment in the second tank, and it is in this compartment that the recirculation pump is suspended so that it is positioned some distance above the bottom, or base, of the tank. Contrary to the depiction in annexure 1, the stilling chamber did not have its own bottom above the bottom of the holding tank.
One misunderstanding said to underlie the trial judge's conclusions is alleged to be expressed in paragraph (d) above. It is common ground that in the BPS System, and also in the modified Parco Beaver System, the waste water which had passed over the disc packs returned to the stilling chamber. However it is said that in the Parco Beaver System this was also the case, not, as the reasons for judgment say, to the outside of the stilling chamber.
The understanding of the way in which the waste water returned to the second holding tank contained in the above description is relevant but not central to the reasoning of the trial judge. As will appear from passages in her Honour's reasons set out below, the most important consequence of Mr Brookfield's modifications related to the change in the filter medium from an aggregate of cut pipes to the disc pack. The trial judge found that all, or nearly all, waste water systems installed by Septic Products utilised the disc pack as the filter medium, and that the preponderance of Doc 3 pumps supplied to Septic Products which were used to replace failed Doc 3 pumps supplied to Mr Brookfield trading as Brookfield Plumbing Services would have been installed in waste water systems which utilised disc packs as the filter medium. These findings are not challenged.
The household waste water processed through these and other aerobic waste water systems on the market includes besides sewage from the lavatory system, sink water, shower water and laundry water. The waste water is not free of solids. Household waste water contains suspended solids of about 250 milligrams per litre, including sand and grit, as it enters the first tank, which acts as a primary settlement tank. Some 60 of those solids are removed by settlement in the primary tank. The balance flows into the second tank where most, but not all, of it will settle before being discharged through the irrigation outlet. Whilst Mr Wilsdon viewed a demonstration unit operating with clean water, he understood that the system processed household waste water. The presence in suspension of solids and grit in the waste water entering the second tank is a feature of the working environment in aerobic septic systems, and submersible pumps employed in these systems must be capable of working in an environment which is not totally free from suspended abrasive solids.
The appellants' case at trial supported by evidence from an expert in pump design, Emeritus Professor P.N. Joubert, was that the shafts of the respondent's pumps were made of steel that was too soft for use in association with the kind of lip seals that were employed to keep the waste water from the electric motor. Failed pumps which Professor Joubert examined showed wear grooves at the location of the lip seals. Motors in the pumps had failed in service as fluid had entered the motor by passing under the lip seals.
The trial judge noted that Mr Brookfield conceded in his oral evidence that most of the failures of Doc 3 and Doc 7 pumps experienced by the appellants occurred in systems which utilised disc packs. In all the systems identified in an extensive schedule to the statement of claim as those installed by the appellants only "a handful" of failures occurred in systems not using filter packs. This finding of fact has not been challenged on appeal.
It is convenient at this point to set out the trial judge's summary of the expert evidence, and reasons as to the failure of the pumps which are critical to a consideration of the appellants' contention that a fundamental misunderstanding has occurred. Her Honour said:
"The only academically qualified expert with experience in waste water process design who gave evidence at the hearing of this matter was Mr Mark Gobbie ('Mr Gobbie'). Mr Gobbie was retained by the solicitors for the respondent to review the design features, performance and operation of the BPS waste water system. In doing so he necessarily gave consideration to the operational significance of using disc packs in lieu of cut plastic piping as a filter medium. Mr Gobbie expressed the opinions, which I accept, that horizontal medium, such as the disc packs, are likely to lead to uneven biomass growth, and that uneven biomass growth will result in large pieces of biomass sloughing from the filter medium into the area of the holding tank in which the recirculation pump (for present purposes, the Doc 3 pump) operates. Although Mr Gobbie assumed, contrary to the fact, that disc packs would not be serviced, I do not consider that this factor affects his above opinions. A view of waste water systems, including a BPS system, was held early in the hearing of this matter. The BPS system seen on the view was one which the evidence established to have been regularly serviced. Uneven biomass distribution on the top disc of the disc pack was pointed out to me and observed by me. Also pointed out to, and observed by me, were lumps of biomass on the surface of the holding tank below the disc pack. What I saw on the view in this regard tends, in my view, to provide support for the opinion expressed by Mr Gobbie (s54 of the Evidence Act 1995 (Cth)).
Mr Wilsdon gave evidence of Doc 3 and Doc 7 pumps returned by Mr Brookfield to the respondent having large quantities of thick waste material in and around the pumps' external workings of the shaft and the impeller. His evidence was that after the removal of such material the pumps operated satisfactorily.
There is other evidence that a number of failed pumps returned to the respondent showed on examination a build up of bacteria growth. Mr Brookfield himself gave evidence of seeing approximately 9 dismantled Doc 3 pumps previously returned by him to the respondent: on each of them he saw a dried substance around the impeller and the pump shaft which, when he rubbed it between his finger and thumb, ground down to a fine gritty substance. His evidence is that he recognised the dried substance immediately as bacteria culture, and that it was identical to the biomass growth which forms on top of filter packs.
The Doc 3 and Doc 7 pumps are manufactured with three lip seals located on the pump shaft. The purpose of these seals is to isolate the motor of the pump from the fluid being pumped ('the working fluid'). Each of the experts in pump design who gave evidence in this case agreed that the Doc 3 and Doc 7 pumps which they had examined which had failed, failed because lip seals located on the pump shafts failed with the result that working fluid entered the motors. The dispute between them was as to why such seals had failed.
Mr John Weir ('Mr Weir'), who was called on behalf of the respondent, expressed the view that the seals failed because the pumps were operated for an extended period in an environment containing an excessively high concentration of abrasive particles. Professor Joubert, the principal pump design expert called on behalf of the applicant, on the other hand, expressed the view that the seals had failed because the pump shafts were manufactured from a steel which was too soft for use with lip-seals. Professor Joubert expressed the opinion, which I have no reason to doubt, that the Doc 3 and Doc 7 pumps tested by him had shafts which were softer than the minimal hardness recommended in an accepted handbook of machine design for shafts that contact lip-seals. However, he made it plain that, so far as wear is concerned, hardness is not the only factor. He pointed out that wear is a very complicated phenomenon which cannot be predicted.
If the Doc 3 and Doc 7 pumps were manufactured with a shaft which was too soft for use with lip-seals in all circumstances, one would expect such pumps to fail irrespective of the use to which they were put. The fact that the Doc 3 pumps failed at a much higher rate than the Doc 7 pumps suggests that some additional factor, or factors, was or were involved. Moreover, as Mr Weir pointed out, one would expect the wear grooves at the location of the lip-seals to be relatively uniform if the problem were one involving no variables other than the nature of the seals and the strength of the shafts. The evidence establishes that on Doc 3 and Doc 7 pumps where the lip-seals had failed, the grooves at the location of
the lip-seals were deepest at the lip-seal closest to the impeller and shallowest at the lip-seal closest to the motor. This gradation of the grooves suggests a relationship between the working fluid and the failure of the lip-seals. Professor Joubert in his oral evidence appeared to accept that such a relationship was likely.
It was not suggested on behalf of either party that there was any difference in the hardness of the shafts of Doc 3 and Doc 7 pumps. Mr Brookfield himself gave evidence of the satisfactory working of Doc 7 pumps in the Parco Beaver system. At the time when he first approached the respondent with respect to purchasing pumps from it, he had been using Doc 7 pumps supplied by Allwater in Parco Beaver systems since 1989. As is mentioned above, he had witnessed extensive testing of Doc 7 pumps by Allwater, apparently in conjunction with the respondent. He had found that the Doc 7 pumps performed satisfactorily in the Parco Beaver system and he was happy to continue to use them. I regard this as compelling evidence that the steel with which the Doc 7 pump shafts are manufactured is not in practice too soft for the satisfactory operation of such pumps in Parco Beaver systems.
Moreover, Mr Wilsdon gave unchallenged evidence of the sale of 22,017 Doc 3 pumps by the respondent between 1988 and 1993 and of the sale of 4,689 Doc 7 pumps over the same period. He was only able to give evidence of warranty replacements of Doc 3 and Doc 7 pumps between 1990-1994. His evidence was that during that period 740 Doc 3 pumps and 297 Doc 7 pumps were replaced under warranty. Whilst there may be reason to doubt that such figures are precisely accurate, I accept that they reflect the order of the number of pumps so replaced. I further accept Mr Wilsdon's evidence that a number of the Doc 3 and Doc 7 pumps sold by the respondent over the periods referred to above were sold to customers other than the applicants for use in waste water systems and that the return rate of such pumps was not out of the ordinary.
...
I find that, on the balance of probabilities, there is a relationship between the established failure of a significant proportion of the Doc 3 pumps supplied to Septic Products by the respondent and the modification effected by Mr Brookfield to the Parco Beaver system. The evidence does not disclose that Mr Brookfield, or any other person on behalf of the applicants, advised the respondent of the modification made by Mr Brookfield to the Parco Beaver system."