text: the various elements identified by numerals in the drawings are listed in the following integer list . referring to fig1 and 2 of the drawings there is shown a septic tank generally designated by the reference numeral 1 which may typically be a reinforced concrete construction . the septic tank includes a circular base 2 and a cylindrical side wall 3 . a lid 5 covers the top of the tank . reinforcement 4 is provided in the base , side wall and lid of the tank . typically this may comprise steel mesh and / or any other reinforcing material which is used for reinforcing concrete . inspection openings 10 and 11 are provided on opposite sides of the lid immediately above the t - pieces 16 and 19 . a large central opening 12 for providing access to the contents of the septic tank when it needs to be emptied is also provided in the lid . this is normally closed by a cover which neatly fits within and closes off the central opening 12 . the side wall is provided with an inlet 15 for sewerage which directs incoming sewerage into the t - piece and hence into the primary chamber 23 of the septic tank . both the top and bottom end of the t - piece are open . an outlet 18 connected to the t - piece 19 takes overflow liquid from the secondary chamber 24 whenever the liquid level 20 exceeds the level of the outlet 18 . the outlet is at a lower level than the inlet . from the outlet , the digested liquid from the septic tank is directed to an in ground distribution system as is known in the art . it is to be noted that there will typically be a scum layer 21 build up at or above level 20 . it may extend higher than the level of the outlet 18 in the region of the primary chamber . the primary and secondary chamber are divided by the dividing wall 22 which has an opening 26 therein at a level below the liquid level 20 . the dividing wall in effect acts as a baffle between the primary and secondary chambers . during typical operation of the septic tank , sludge builds up in the primary chamber to a sludge level which is indicated by the line 27 . over time the sludge , which is of greater density than water builds up from the floor of the tank 4 until it begins to approach the level of the opening 26 . however , it is important to ensure that the sludge level does not go so high as to allow the sludge to overflow into the secondary chamber 24 as this will mean the residence time of sewerage in the septic tank has been reduced and there is a risk that inadequately digested fluid will pass through into the secondary chamber and out of the outlet . as the sludge initially has a higher density than the water , an indication of the level of sludge in the primary chamber can be obtained by locating a pressure sensor 28 near the base of the septic tank . the pressure sensor is connected by cabling 29 to a monitor system 30 . the monitor system may include electronics to take account of the electric pressure signal provided by the pressure sensor 28 either continuously or on a periodic basis . when the pressure rises above a predetermined limit as may be set on the monitor system , the monitor system may generate a warning or other indication that the septic tank needs to be inspected and possibly emptied . for example , if the pressure of fluid including sludge in the primary chamber increases by as much as 1 % or 2 % above the pressure which would be expected with pure water , the monitoring system may generate an appropriate signal to indicate that the tank needs to be inspected . in order to do so , the monitor system may include a reference point setting for adjusting the pressure sensitivity of the monitoring system for different sizes of tanks ie . a deeper tank will have a higher reference point pressure than a shallower one . thus for each tank , an individual pressure based upon a primary chamber filled with pure water may be set as a reference point . the graph in fig3 indicates a typical output from the pressure sensor registered by the monitor as readings of sludge depth over time . the actual readings can vary but they are then treated with a regression algorithm to create a line of best fit . this is then used to monitor trends and predict trigger points for service . typically a pressure rise of about 1 % may be used as a trigger point for service , as this may be indicative of a depth of sludge which has reached the limit of acceptability . however , the actual trigger point may vary depending on the configuration of the septic tank . another way of looking at the trigger point would be to equate it to a predetermined acceptable level of sludge . for instance in a septic tank with a dividing wall 22 , it may be preferable to set the trigger point at a pressure level in the primary chamber which corresponds to a sludge depth at or below the level of the opening 26 . in another scenario , if the pressure sensor is placed in the secondary chamber , any consistent increase in pressure may be sufficient to trigger a warning . in an alternative scenario , applicants have also found that after a period of time , following an initial rise in pressure in the primary chamber , there may also be a reduction in pressure . whilst the reasons for this reduction in pressure are unclear , on the basis of experiments carried out so far , it would appear that the change from increase in pressure to a reduction of pressure of 1 % may be taken as a trigger point or warning point whereby the monitor system generates a signal to indicate that the septic tank needs to be emptied . similarly , an additional pressure sensor may be placed at or below the scum layer . again this may be set so that a pressure variation of about 1 % triggers an indication that an inspection is required . whilst the above description includes the preferred embodiments of the invention , it is to be understood that many variations , alterations , modifications and / or additions may be introduced into the constructions and arrangements of parts previously described without departing from the essential features or the spirit or ambit of the invention . it will be also understood that where the word “ comprise ”, and variations such as “ comprises ” and “ comprising ”, are used in this specification , unless the context requires otherwise such use is intended to imply the inclusion of a stated feature or features but is not to be taken as excluding the presence of other feature or features . the reference to any prior art in this specification is not , and should not be taken as , an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge in australia .