FIELD SEPTIC DISPOSAL SYSTEM (FSDS) CERTIFICATION STANDARDS COMPLIANCE AS/NZS :1998 REPORT

Transcription

1 FIELD SEPTIC DISPOSAL SYSTEM (FSDS) CERTIFICATION STANDARDS COMPLIANCE AS/NZS :1998 REPORT

2 1.0 INTRODUCTION 2 My name is Robert Brown, B.Sc. Civ.Eng., and I have been engaged by Humanitarian Environmental Solutions Pty.Ltd. of Port Macquarie N.S.W. to investigate the Field Septic Disposal System (FSDS) for compliance with the Australian and New Zealand standard :1998. On-site domestic wastewater treatment units. Part 1: Septic tanks. I am satisfied that the FSDS has been developed to achieve the Performance Standards required by the Australian and New Zealand Standards over a wide range of situations giving the wastewater system designer a number of new and innovative design options. The FSDS is essentially a septic tank designed for the primary treatment of toilet wastes (black water), sullage wastes (grey water) or a combination of both (dual purpose). The key difference between FSDS and other septic tank systems is the new and innovative philosophy of designing a number of suitably sized smaller FSDS primary treatment systems rather than simply connecting all sanitary fittings to a single large conventional septic tank. Individual FSDS units are provided for each group of sanitary fittings. The sum total of primary treatment capacity provided by a number of FSDS will equal or exceed the total primary treatment capacity provided by a single larger conventional septic tank. The FSDS has been manufactured from mainly plastic and vinyl products. The FSDS components have been developed so that they are available in pack form for easy transport to site and then be assembled on-site with a minimum of equipment. FSDS is manufactured in accordance with a project s specific wastewater design requirements and are produced in a range of sizes suitable from individual or small groups of sanitary fittings to large installations. Blackwater and greywater sanitary fittings can be kept separate to facilitate greywater treatment and recycling. FSDS produces primary treated water only and requires an approved means of secondary treatment designed in accordance with (AN/NZS 1547: 2000 On-site domestic-wastewater management) to be provided downstream of the FSDS. Individual or groups of FSDS chambers kits may be used to provide an effective means of wastewater disposal during civil emergencies or natural disasters. The number of individual FSDS chambers required being determined by the population being supported and the available water supply. FSDS provides the required primary treatment for the Evapocycle TM system.

3 STANDARDS REQUIREMENTS 3 AS/NZS :1998 covers the performance requirements and performance criteria for septic tanks. Section 2 sets out these requirements and criteria and Sections 3 to 10 cover the means of compliance for septic tanks constructed of various materials. None of the Sections deals with the materials from which FSDS is manufactured. Clause of the Standard allows for septic tanks constructed of other materials to be assessed under the requirements and criteria of section 2. This report should be read in conjunction with section 2 of AS/NZS as the full text of the standard is not reproduced here. Clause numbers refer to the clause in the Standard. Clause 2.2 Function and Context of Use Clause FUNCTION The function of FSDS is to provide a relatively still zone of adequate size for the treatment of domestic wastewater at all flow rates from a household, or institutional or commercial facility. Scum, and solids capable of settling, are separated from the wastewater flow. The placement of inlet, transfer, and outlet pipes within the FSDS allows sewage solids to be retained, digested, and consolidated between maintenance operations. The FSDS design allows the liquid above the settled solids (sludge) and below the scum layer to be discharged to a land application system. Clause CONTEXT OF USE The FSDS is designed to be installed in the ground in a location such that the water table cannot rise above the level of the outlet pipe. Clause 2.3 Performance Requirements Performance standards are set out which are then met by the following criteria. Clause 2.4 Performance Criteria. Recommended minimum capacities for conventional wastewater treatment units are given in Appendix B(AS/NZS ). This has now been replaced by AN/NZS 1547:2000 On-site domestic-wastewater management. This Standard has a broader scope than previous Standards and includes: (a) Performance statements necessary to define outcomes and to accommodate new technologies, (b) Provides the basic performance provisions for septic tanks (AS/NZS ) and introduces performance requirements to cover all types of wastewater treatment units and land application systems.

4 Innovation and alternative materials systems and methods. AS/NZS 1547:2000 does not preclude the use of any material, system design or method of implementation provided the completed system and installation meet the performance requirements of that Standard. 1 4 The FSDS is an innovative approach to domestic wastewater management. The required performance standards are achieved through the use of a range of different sized FSDS units that are specifically designed capacities to provide the necessary treatment (performance requirements) of particular groups of sanitary fixtures within a development. This approach allows a high degree of wastewater design flexibility which results in more effective methods of wastewater management. FSDS Design Procedure. The wastewater treatment system shall be designed to cater for the number of people for which the premise is designed and its intended use. Typical Domestic wastewater flow design allowances in L/person/day are shown in AS/NZS 1547:2000 Appendix 4.2D. Factors that affect these flow design allowances include; (i) Whether the water supply is on-site roof tanks or a reticulated community or a bore-water supply, (ii) Whether standard or full water-reduction fixtures are to be used, and (iii) Water reduction through the treatment and recycling of grey water. The total wastewater design allowance is then apportioned to selected groups or clusters of sanitary fixtures that have been identified by the FSDS system designer as appropriate. Factors that affect this decision include; (i) (ii) The location of the fixtures within the building(s) Whether is intended to provide greywater treatment and recycling. The Cluster wastewater design allowance is the determined based on the type of sanitary fixture and population. The FSDS system designer will then designate the actual primary treatment intended for each cluster. This may include the use of a FSDS (blackwater), FSDS (greywater), FSDS (allwaste) units or a combination of one or more in accordance with specific cluster design requirements. Clause DESIGN CAPACITY The size (capacity) and type of each FSDS for each cluster is then determined as follows: 1. FSDS (blackwater) units. Volume will be determined by the requirement to achieve a 24-hour settling volume and an additional capacity to allow for sludge accumulation over a 3 to 5 year period. The blackwater sludge accumulation is 50L/person/year. An adjustment is made to the sludge accumulation volume 1 AS/NZS 1547:2000 Clause 1.4.2

5 5 according to the areas average temperatures that directly affect the actual rate of sludge digestion. 2. FSDS (greywater) units. Volume will be determined by the requirement to allow for approximately 24-hour settling volume plus an allowance of 8 hours hydraulic buffering volume for the daily greywater flow. An additional capacity to allow scum accumulation over a 3 to 5 year period. The greywater scum accumulation allowance is 40L/person/year. 3. FSDS (All-waste) units. Volume will be determined by the requirement to allow for approximately 24-hour settling volume and an additional sludge and scum storage capacity to allow for sludge and scum accumulation over a 3 to 5 year period. An adjustment is made to the sludge accumulation volume according to the areas average temperatures that directly affect the actual rate of sludge digestion. The sludge and scum accumulation allowance is 80L/person/year. Clause FLOW PATH Where the size of an FSDS restricts the flow path from inlet to outlet is less than 1200mm a second FSDS compartment is installed. The minimum distance between each compartment will be not less than 2m. Clause INLET AND OUTLET FITTINGS. Inlet and outlet fittings are minimum 100mm diameter which is considered adequate to meet this criterion. Clause JOINTS Joint seals have been tested and found to be watertight. The test was conducted in accordance with the requirements of Appendix C (AS/NZS ). Clause PARTITIONS Partitioning is provided by separation of compartments connected by pipe work. Clause ACCESS OPENINGS AND COVERS Access openings and covers are provided above each inlet and outlet fitting using 100mm diameter pipes and covers. Access openings are provided through the top covers to allow access for desludging of the compartments. Access covers are durable and able to withstand superimposed loads and designed to prevent removal by children. Clause INSPECTION OPENINGS AND COVERS Inspection is normally through the same openings as in The area of a 100mm diameter pipe is 7,800 mm2 which exceeds the 7,500 mm2 required. Inspection can also be achieved by removing the access cover if necessary. Other parts of this criterion are considered to be met.

6 Clause EXTENSIONS 6 The top of the FSDS is set at least 50mm above ground level. There fore this clause does not apply. Clause WATERTIGHTNESS A FSDS compartment (external carcass and STEP liner) was assembled and tested for watertightness. The compartment had outlet capped and was then filled with water. It was inspected three times over a 24 hour period and there were no reported leakages. This test exceeds the requirements of appendix E in that the tank was filled above the outlet pipe. Clause INTEGRITY No structural defect occurred during the watertightness test. Sub-clauses do not apply. Clause LOADS ON TANKS Clause General The FSDS units are designed to resist the forces likely to be imposed on them. Clause Integrity During Handling and Installation The FSDS is designed as a lightweight easily transportable unit. A compartment pack is delivered in a cardboard carton for transport in a light vehicle. Pipes and timber covers are similarly easily transported. If concrete covers are used these would require lifting equipment but are designed to withstand handling. Larger FSDS installations use a suitably sized STEP kit liner only with the external supporting structure being constructed from masonry components on-site. Clause Hydrostatic Uplift. The liquid containing part of the FSDS compartment is the STEP kit liner that is essentially a flexible bag. The outer carcass component or masonry supporting structure for the larger installations of the FSDS that supports the STEP kit liner has no base and is not water proof, hence the hydrostatic load applies only to STEP kit liner. The FSDS must be installed so that the water table cannot rise above the outlet pipe otherwise the STEP kit liner could float. In practice this is not a problem as the entire surrounding area would be flooded in this case and water would flood into the unit. The same applies to all other septic tanks installations. Clause Lateral Loads The outer carcass component of the FSDS can only be subject to soil loads rather than the saturated soil and water load to which a conventional septic tank could be subjected. Nevertheless the structure has been tested to the loading of 6.6kPa/m depth suggested in the Standard. The FSDS unit is never set at a greater depth than the depth of the unit as the top is set at least 50mm above ground level. The total design load calculated to apply on a single prefabricated outer carcass section is 56 kilograms at the standard loading rate. A FSDS carcass section, in the assembled condition was subjected to a distribution load of

7 approximately 75 kilograms. There was no failure or permanent deformation of the panel or of the joints. The unit is considered to have met this criterion. Clause Top Loads. The top of the FSDS can be either 19mm structural plywood or precast concrete. Both these have been analysed for the capacity to resist an applied load of 5 kn applied on a block 250mm square. The plywood, of strength grade F11 or better can withstand this load under test load conditions in accordance with the Australian Standard for timber structures. 7 Cast-in-situ concrete tops should be 125 mm thick with F81 fabric reinforcement to provide adequate cover to steel for fabrication in the field. AS?NZS 1546 would allow a minimum of 75mm thickness for factory produced tops. Reinforcement would be F81. In both cases the tops would resist the imposed loads. Tops are designed to be supported on the surrounding, not on the carcass structure. Flexible pipe joints provided in the STEP kit liners are designed to accommodate any small settlement. 3.0 CONCLUSION The FSDS system is a unique and innovative method of on-site domestic wastewater management. The FSDS complies with both the Australian and New Zealand Standards Performance requirements, Performance criteria, construction and materials requirements of AS/NZS :1998 On-site domestic wastewater treatment units) and the capacity and management requirements of AS/NZS 1546:2000 On-site domestic- wastewater management. When correctly installed the FSDS achieves the required standard of primary wastewater treatment and provides the required sludge and scum retention capacities on a decentralised basis depending on the population based wastewater flow design allowances for a specified group or cluster of sanitary fixtures. This allows wastewater system designers greater scope to adopt new and innovative wastewater management options including greywater recycling. Robert Brown, B.Sc. Civ.Eng., Beaconsfield Road, Portobello, Dunedin, New Zealand. 30 May, 2007.