R.M.C. Patterson, Environmental Consultant (2001).


ABSTRACT

Urban populations must have basic sanitation and potable water infrastructures. In disrupted states or during other civil emergencies such as natural disasters, these infrastructures are usually made non-operational and are therefore effectively removed. Disruption of basic hygiene and sanitation, together with an increased exposure to hazards and reduced resistance to disease are identified by the U.S. Manual of Naval Preventative medicine as the three main reasons why disease has always caused greater personnel loss in military forces than enemy action. Personnel operating in situations where basic sanitation and water supplies are failing or non-existent are at greater risk of contracting a communicable disease than in garrison. Poor sanitation and water supplies can have a significant impact on the ability of that military unit or similar organisation to perform its role. The diseases to which military forces and other non-government organisations are subjected are largely preventable or controllable.

The aim of this paper is to examine the effectiveness of new wastewater disposal techniques that were used by the International Force East Timor (INTERFET) to safely dispose of human waste in urban areas of DILI and other remote areas such as SUAI on the boarder with West Timor, that have failed, disrupted or non-existent infrastructures.

There were a limited number of sewerage waste disposal options available to INTERFET during the initial deployment to permit the occupation of the disrupted urban areas. The use of an option or a combination of options was dependent on factors such as the size of the force or organisation; the length of time before basic infrastructure will be operational, environmental factors such as climate, soil types, high water tables and the tactical situation. Options included individual cat scrape, shallow trench latrines (SLT), portable toilets, and water sealed field toilets connected to field septic tanks (FST). Water sealed toilets connected to FST requires the use of absorption trenches to provide the required secondary treatment.

Land based sewage disposal options for urban and high troop or displaced person concentration areas were considered. These options fall into two basic categories – centralised systems and on-site systems. Centralised systems involve the use of the existing sewage treatment infrastructure to collect the effluent for treatment and disposal off site in a single large facility. Treated effluent from the centralised facility is then preferably directed to a land based system for secondary treatment and disposal. On-site systems provide both primary and secondary treatment and disposal within the property boundary or immediate area. The treatment and disposal processes are essentially the same.

The fundamental problem of land based secondary treatment is the ability of the ground to treat and dispose of the large volumes of primary treated effluent. Historically land based absorption trenches have had a poor performance record. This in part is a result site limitations such as non-absorbent soil types or high water tables that cause the traditional or standard absorption trenches to fail. This problem has been examined in detail and over a number of years has resulted in the development of the Modified Absorption Trench (MAT) systems. These MAT systems are designed to eliminate site limitations as a factor that can interfere with the natural physical and biological treatment processes that occur in ideal soil types. The development of MAT has enabled ground absorption in soils with poor permeability, (less than 0.05m/day) or highly permeable soils (greater than 0.6m/day).

MAT have been further developed in Tasmania to also be suitable in areas with very high seasonal water tables, shallow soils over rock and high rainfalls that exceed evaporation rates. There are a number MAT systems operating in commercial and domestic developments around Tasmania in areas with extreme site limitations that were previously considered impossible for on-site wastewater disposal.

The Australian Defence Force was aware of these developments and the need for effective wastewater treatment and disposal to ensure INTERFET sustainability in East Timor. The author who is a member of the Australian Army reserve was transferred to full time duty and arrived in Dili on 4th October 1999.

The following Case Studies will be discussed in this paper.
1. Wastewater disposal for the Royal Australian Air Force at Komoro Airfield (Dili), and
2. Wastewater disposal for the Royal New Zealand Air Force (RNZAF) and Royal New Zealand Army (RNZA) units at Suai near the boarder with West Timor.

KEYWORDS

FSDS, MAT (E), INTERFET, Absorption trench, on-site wastewater disposal.

1. INTRODUCTION

The objective of environmental sanitation is the prevention of the transmission of communicable disease. This is carried out by controlling the source, the route and the target. Achievement of the following basic objectives will significantly reduce disease caused by human sewage:
a. Prevent access of insects and rodents to sanitary facilities and treatment systems.
b. Prevent transfer of faeces and urine from person to potable water or food.
c. Prevent disease contamination of surface water or ground water.
d. Prevent contamination of the soil surface

While on-site wastewater disposal systems are capable of meeting the above objectives; they have historically been seen as a temporary measure only until centralized sewage treatment could be provided.

The basic philosophy for the traditional on-site wastewater disposal system was that a septic tank system using a standard absorption trench would make the wastewater go away. This philosophy was seriously flawed because many soil types are non-absorptive which restricts the infiltration of the effluent through slow percolation. In those circumstances many septic systems failed with trenches filling up and seeping to the surface. Seeping trenches greatly increased the potential for the spread of disease and water pollution after rainfall.

Standard trench failure would also result from flooding caused by seasonally high water tables and shallow bedrock. Rapidly percolating sand and gravel soil types were also a problem due the potential of untreated wastewater contamination of ground water supplies
.
The problem of sewage wastewater disposal was further complicated by the wide spread failure of centralized sewage treatment systems that were causing serious pollution of the receiving waters into which they discharge.

The early 1980’s saw a refocusing of Health and Water Authorities around the world on to the soil as an efficient natural wastewater treatment and disposal mechanism. The challenge was to identify those factors that restricted the use of soil as a treatment mechanism and develop techniques to defeat those restrictive factors.

2. ABSORPTION TRENCHES OPTIONS

In Tasmania effective septic tank effluent disposal is achieved through the use of three basic designs with site specific modifications being applied as required.

Standard Absorption Trench. (SAT)
The SAT requires the following site conditions;

- Sandy loam to clay loam soil types
- Minimum 1000mm of soil.
- No soil limitations within 1 metre of the base or sides of the trench.
- Rainfall is less than or equal to evaporation rates in any one-month.

SAT systems are not suitable in rapidly percolating soil types.

Modified Absorption Trench (in ground) MAT (G).
This trench type is used in areas with shallow topsoil or rapidly permeable soil types.

MAT (G) are suitable in the following site conditions;
- Any soil type except clay
- Minimum 200mm of sand or sandy loam as topsoil.
- No soil limitations within 1 metre of the base or sides of the trench
- Rainfall is less than or equal to evaporation rates in any one-month.

In rapidly percolating soil types such as coarse grained sand and gravel MAT (G) are constructed with a suitable barrier or clogging mat at the base of the trench.

Modified Absorption Trench (Elevated) MAT (E).
Mat (E) are used in areas with no or shallow soils less than
200mm and/or high seasonal water tables.

MAT (E) are suitable in the following site conditions.
- All soil types including impermeable soils such as clay.
- A soil limitation may occur within 1 metre of the base or sides of the trench if an impervious barrier is fitted to the base of the inner core.
- Rainfall may exceed evaporation rates in any month if fitted with an impervious barrier over the top and sides of the inner core.

MAT (E) usually require pressure distribution, which enables the effluent to be evenly distributed along the entire length of the trench. This avoids the problems of localized overloading characteristic of some gravity systems. Periodic dosing also permits aeration at the soil interface between dosing.

MAT (G) and MAT (E) techniques allows the soils natural treatment mechanisms to operate free from limiting factors. The effective secondary treatment provided by these systems has enabled reconsideration of the simple septic tank as a primary treatment option.

3. CASE STUDIES

The problem faced by INTERFET in both case studies was to provide adequate sanitation to ensure force sustainability. The two sites selected are remote locations with site limitations. The use of the basic shallow trench latrine systems (SLT) can only be considered for short periods of time during the dry season. The supply and maintenance of portable toilets was also not a sustainable option.

CASE STUDY 1.

Wastewater disposal for the Royal Australian Air Force (RAAF) at Komoro Airfield (Dili).

The RAAF required the provision of permanent toilet facilities for use by airfield ground defenders who had established strong points to protect the northern and eastern approaches to the Komoro runway.

The area generally is flat with many low-lying flood prone areas that were expected to become inundated during the wet season. There was no power supply and the use of field generators was not an option due to the tactical situation. Limited equipment including a backhoe with a front-end loader was available on a limited basis.

Suitable locations were identified at each site. Each location was the highest point possible. The site on the northern side of the runway was on a road shoulder. Primary treatment was achieved using a three-chambered Field Septic Disposal System installed at each location. Three chambered systems are suitable for up to 25 persons and due to the high digestion rates desludging intervals were expected to exceed six months.
Two standard WC pans were positioned directly above the first two chambers. This removed the need for additional plumbing. Cisterns were also replaced with the FSDS pour flush funnel that replicates the function of a standard cistern without the bulk or operating mechanism. To operate the system4 to 6 litres of water is poured into the flush funnel connected to the WC. Prior to pouring the water is used for hand washing. The flushing action evacuates faeces and paper from the WC and retains the water seal to prevent the access of insect vectors.

The FSDS has been designed to function in the same manner as a standard septic tank. FSDS have been certified as complying with Australian / New Zealand Standard AS/NZS 1546.1: 1998, On-site domestic wastewater treatment units, Septic tanks. Solids are removed from the waste stream and digestion of those solids is facilitated. Unique features of the system that make them ideal for military or emergency deployments are its light weight construction (15Kg) for each chamber, its ability to automatically adjust to seasonally high water tables and ability to cater very low or very high shock loads.

Secondary treatment of the wastewater was achieved in 5m MAT (G) constructed from locally available materials.

Both systems operated without maintenance from November 1999 to March 2000. The transition from INTERFET to the United Nations Transitional Administration East Timor (UNTAET) and an improved security situation generally caused a reduction in use of these facilities. FSDS installations have a certified life of 15 years and the Komoro Airfield installations will remain in place for use as required in the future.

CASE STUDY 2.

Wastewater disposal for the Royal New Zealand Air Force (RNZAF) and Royal New Zealand Army (RNZA) units at Suai near the boarder with West Timor.

RNZAF and RNZA units also required the provision of permanent toilet facilities. These facilities were provided in a similar manner to those discussed in CASE STUDY 1. An additional requirement was for the collection and on-site treatment of sullage wastewater generated from shower, laundry and kitchen facilities.

The area generally is flat with many low-lying flood prone areas that were also expected to become inundated during the wet season. Power supply was available from field generators 5 HP petrol pumps were also available. The only available equipment was the occasional use of a BobCat front-end loader.

The nature of the soil (medium to heavy clays) and the larger volumes of wastewater that would be generated required the use of a suitably sized MAT (E) system. The key problem with this project was the requirement to provide a sullage collection well adjacent each of the shower, laundry and kitchen facilities from which the sullage could be pumped to the MAT (E) system. Suitably sized collection wells have capacities of between 2000 L and 5000 L and need to be located below ground to enable drainage from floor level. Only plastic prefabricated collection wells could be transported to SUAI and apart from the expected time delay of between 4 to 6 weeks, plastic tanks are not suitable for below ground installation in heavy clay soils with high water tables. The continual emptying of plastic collection wells causes significant hydrostatic lifting forces when there is a high seasonal water table. The provision of suitable anchorage for plastic collection wells under these conditions is also difficult.

The collection well problem was solved simply using locally available materials. A trench 10m long, 1.2m wide and 1.2m deep was constructed at the rear of each facility. Each trench was lined with black plastic (double thickness) and covered with a lightweight roof positioned on a double layer of sandbags. The lightweight roof was also covered with black plastic (double thickness) to protect the trench from rainwater infiltration. The roof black plastic was held in place with 20mm to 40mm of backfill soil. The construction of the lightweight roof also prevented access by insects such as mosquitoes that would have increases the risk Malaria or Dengue Fever. Drainage pipes from the facilities were connected into these trenches.

The operation of the trench is also simple. Sullage is collected and pumped as required to the MAT (E) absorption system. The 10m trench has a holding capacity of over 14,000-L, which is approximately 14 days capacity for 200 persons. During the wet season when the water table will rise to within 600mm of the surface the actual storage capacity is reduced by 50% to approximately 7000 L. The reduced capacity is caused by the requirement not to pump out more sullage than the corresponding water table, which allows the trench to remain in equilibrium with the water table. If additional sullage is removed below the level of the water table the hydrostatic force would caused the plastic lining on the sides if the trench to fail.

4. LESSONS LEARNT.

The development of MAT (G) and MAT (E) absorption has revolutionised on-site secondary treatment of sewage wastewater. This development has permitted the installation of water sealed toilets connected to FSDS or basic septic tank system in areas previously thought impossible due to non-absorbent soil types or other site limitations.

The development of the FSDS for use in conjunction with MAT (G) or MAT (E) has allowed the use of water sealed toilets in remote areas. Systems have also been identified as suitable for NGO use in disrupted states and other civil emergencies.

The low comparative costs of these on-site sewage wastewater systems will allow the high levels of sanitation infrastructure in developed countries to be provided to third world countries.

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