FIELD OF THE INVENTION[0001] The present invention relates in general tothefield of filter devices for filtering liquids containing par-ticulate matters, more specifically waste water. Moreparticularly, it concernsa filter unitforuse ina filter hous-ing adaptedtobe mountedatthe outlet ofa septic tank.It also concernsa combination ofa filter unit witha filterhousing suitableforuse ina septic tank, such combina-tion being hereinafter calleda prefilter.BACKGROUND OFTHE INVENTION[0002] The development of prefilters to favourthe re-tention of solid matters inside septic tanks goes backabout 40 years (US patent no.2 900 084). Such prefil-ters have spread duringthe last20 years, and more par-ticularly duringthe last 10 years. As an illustration, manyAmerican states legally require the installation of prefil-ters atthe outlet ofthe tanks, in orderto limit the chargeof particulate matters to be treated atthe purifying ele-ment following the septic tank.[0003] Already known in the prior art, there are prefil-ters composed ofoneor many hollow vertical elementsof various forms perforated with openings or slits andenclosed ina housing having an inlet for receiving thewaste waterand an outlet connected tothe outlet oftheseptictank in orderto allowthe evacuation ofthe filteredwater. Examples of such prior art prefilters are given inUS 4,439,323; US 5,492,635; US 5,580,453; US5,382,357; and US 5,482,621. Usually, these prefiltersdo not provide decantation and/or coalescence phe-nomenon ofthe finer particles. The retention is thus as-sured essentially by the chosen size of the openingswhether they be orifices or slits. The feed of these ver-tical filtering structures can be done from the interior ofthe element towards the exterior or from the exterior to-wardsthe interior, depending on the selected hydraulicflow model. The recourse to such hollow vertical struc-tures favoursa partial detachment ofthe biomass butrequires eithera filtering element of large dimension, asdisclosed in US 4,439,323 orthe multiplication of thenumber of filtering elements, as disclosed in US5,492,635, and US 5,580,453. If this is not the case,thetotal available filtration area requires more than onecleaning per yearfor residences producinga dischargecorresponding toan occupation of more than four peo-ple.[0004] Also known in the prior art, there are the pre-filters of the type disclosed in US 4,710,295 which in-cludea pt urality of stacked horizontally extending disc-dam units mounted in vertical ina housing. Each disc isequipped with finger shaped appendices or diverticu-tums formed bya continuousdam wall that followsa sin-uous path and whosetop side is open. The superposi-tionand the alignmentofthe discs by ensuringa spacing10203040502stice and the sinuous dam-wall createsa second filtra-tion interstice. Therefore, fora given volume, it is pos-sibleto increasethe available interstitial surface in com-parison withthe one corresponding tothesum ofthesetof slits or perforated orifices ina hollow vertical elementas described above. In such prefilters, the water tobefiltered entersthe housingand flows upwardly in verticalchannelsand then inthe horizontal channels formedbe-tween each disc where it is filtered. Thus, the control ofthe particles to be retained is done first at the first filtra-tion interstice. The water then flows horizontally and isfurther filtered by the dam wall. Usually, the larger par-ticles are retained on the outside of the disc-dam unitf5and the finer particles have the possibility of decantingin the zone beMeentwo horizontal discs leading totheoutlet zone.[0005] One drawback encountered with such prefil-ters is that with time, the accumulation of fine particlesand the formation ofa biological film between the hori-zontal discs creates hydraulic restrictions that favouradetachment ofthe accumulated matters towards the ef-fluent ofthe septic tankand ultimately requiresa clean-ing. Cleaninga structure of this type is tedious and may25requirea complete dismantling ofthe different elements.[0006] Furthermore, although such prefilters offerabetter efficiency than whatwas known previously, onegoal in this field is still to uncover or developa prefilterbeingthe least voluminous possible and whichcan offerthe maximum filtration capacity.[0007] Therefore, there is stilla need atthe presenttimefora filter witha higher efficiency than the existingfilters.[0008] Other examples of prior art filter apparatuses35for waste water are given in: CA 2,135,937; US3,332,552; US 5,582,716; US 5,593,584; and US5,683,577.SUMMARYOFTHE INVENTION[0009] An objectofthe present invention isto proposea filter unit that satisfies the above-mentioned need.[0010] According tothe present invention, that objectis achieved witha filter unit ina filter housing hereinafter45referred to asa prefilter as defined in the appendedclaims. The filter unit comprises superposed andspaced-apart inclined IameIIar structures each havingalower end side opposite an upperend side; and a pas-sage fora flowof liquid betweeneachtwoofthe IameIIarstructures. Each passage has an inlet for receiving aninflowof liquidto be filtered and an outletfor dischargingan outflow of filtered liquid. The filter unit further com-prises filtering means in each ofthe passages forob-structing the flowof liquid and retaining particulate mat-55ter contained in the liquid. Mounting meansfor mountingthe filter unit vertically in the filter housing are also pro-vided.[0011] Advantageously, the inclined configuration of
