2.4 APPROPRIATE TECHNOLOGIES IN WATER DISPOSAL SYSTEMS

 

2.4.1 INTRODUCTION

 

This proposal for wastewater management systems for small communities in Namibia is based on 'Namibia Consult Incorporated's' feasibility report for different solutions for the R.C. Church missionary station of Anamulenge in Northern Namibia (Ref. 88/4 dated March 1989).

Wastewater disposal may well be the 21st century's world war in industrialised as well as semi-developed regions. The latter ones are still plagued with the handling of water-borne waste in volumes too small to warrant conventional sewerage treatment plants. Worldwide civil engineers have been developed different systems to solve the problem. Sewage effluents are passed through complicated, conventional sewerage purification systems, stabilisation ponds or reed beds, planted in a drainage substrate of ash, gravel or other materials, where oxygen transfers occurs via the stems and rhizomes. This promotes the growth of bacteria to break down sewerage in the same manner as in conventional treatment. But all these systems have one in common: They have a relatively high failure rate, are relatively expensive and difficult to build and to maintain. As for so many other civil engineering tasks in the developing world it should be strived to keep the systems simple, planned and designed for informal settlements tackling pertinent local problems on grassroots level.

But, it should also be strived to conserve, re-cycle and re-use the limited water resources available, particularly in a semi- arid country like Namibia. Agricultural crop irrigation could be one of the schemes to re-cycle treated wastewater where it can be economically justified. But, it should also be seriously considered to re-approach the implementation of flush toilet systems. In many cases these schemes failed due to a lack of proper management and insufficient maintenance. Unfunctional flush toilet are not only wasting water and overloading underdesigned sewerage systems but can also be a greater nuisance than pit latrine systems. Innovations like the 'Ventilated Improved Pit Latrines "V.I.P."' should be seriously looked at.

Different simple solutions to develop wastewater disposal systems will be investigated in this study. Unconventional methods have to be followed because the population in many Namibian communities is too small to justify conventional purification of sewerage and the available funds are limited. Land in most cases is freely available and inexpensive and the organic loadings fluctuate widely due to the variant nature of the occupancy rate. Four different solutions for the sewerage treatment systems were investigated and will be dealt with in the next sections of this report. The investigations were based on a real feasibility study for a community in northern Namibia.

 

2.4.2 APPROPRIATE WASTEWATER MANAGEMENT SYSTEMS

 

The present production of sewage effluent can be estimated on the ground of the daily water demand of a system. The biochemical oxygen demand for five days B.O.D.5 is the organic load which is the O2 quantity which is required to oxidise the raw wastewater influent by means of bacteria. 'B.O.D.' is the dimensioning factor to establish the size and type of any sewerage treatment system. This treatment should be based on a low cost system to handle these domestic wastewaters. The function of the treatment relies on the natural self- purification process - basically photosynthetic oxidation - that occurs in a body of water and is dependent on factors such as sunshine, temperature and wind action. The objective of sewerage systems is to produce an effluent of acceptable quality and not to create a nuisance at reasonable costs. A nuisance-free system is one that does not produce undesirable odours, favour mosquito and fly breeding as well as is no threat to public health and will not degrade the environment significantly.

In many cases the conservancy tank effluent is pumped direct to the surrounding area or via loose PVC pipelines to irrigation areas some distance away where the conservancy tank effluent is distributed, is seeping away or evaporates. This practice causes bad odours, nuisances and can even cause pollution and unhygienic conditions if it is not properly planned and designed for such conditions. The next sections deal with a factual case study with different technical and economical alternatives.

 

2.4.2.1.1 PRIME COSTS

 

(i)   Second-hand tanker unit 80 PS tractor with 5 000 l trailer and        mounted suction pump R 50.000

(ii)  Sewage pipelines to unconnected sewage points 100 m @ R 35
      R 3.500

(iii) Evacuation basins 2 basins 50 m x 10 m x 1 m R 12.000

(iv)  Fencing around evacuation basins 150 m @ R 20 R 3.000

(v)   Construction costs escalations, general contingencies and
      professional fees 40% R 27.400

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Total project prime costs : Solution 1 R 95.900

 

2.4.2.1.2 DEPRECIATION AND RUNNING COSTS

 

(i)  Tanker unit per day (Depreciation over 5 years) R 90

(ii) Labourers 2 per day R 50

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Total per day R 140

Total running costs per year (260 days) R 36.400

The present worth of total net costs in 40 years at 10% p.a. is R 621.908,00 for a salvage value of R 15.000. This solution requires small project costs but high running costs. It is also an disadvantage that the wastewater has to be treated in evacuation basins which will be outside the grounds of the station because inside there will be no space for it and to guarantee a nuisance-free operation. Permission from the local authorities has to be obtained to purchase the required land for the basins.

 

2.4.2.2 SOLUTION 2 : OXIDATION PONDS

 

This solution needs the construction of 1.200 m new underground sewage pipes with 12 additional manholes and 1 new conservancy tank north of the sister building. In order to fit to the extreme small natural slopes it will be required to build the secondary and main sewer lines with minimum grades. This is also required to minimise the excavation quantities of the pipelines. Depending on the survey for the detail design for the sewerage system it is proposed to use an average slope of 0,005 (1:200). If it should be found that this small slope results in pipe blockages (by the usages of plastic bags and other misusages) it would be possible to construct subsequently a flushing tank at the end of the system which could be activated twice a day. In the case that these minimum slopes cannot be achieved it will be required to include a pumping system in order to pump the waste water into the pond system. It is suggested to operate two sewage pumps, one diesel and one electrical pump. The two pumps will be installed at the new conservancy tank north of the sister building. This conservancy tank has to be dimensioned in order to accept the sewerage for four days (80 m3), in order to avoid pumping at public holidays.

The purification of wastewaters will be achieved by means of unventilated oxidation ponds. Such a pond system can be designed to treat conservancy tank contents. Pond systems are simple and economical to construct, operate and maintain. It must be stressed that such system requires proper planning, design, construction and maintenance and a periodic review with regard to pond loading. The ponds have to be located according to the natural slopes and to keep it outside the prevailing wind directions in order to minimise any nuisance to the community.

The size of the primary pond is established by the B.O.D.5 concentration in the raw water influent into the first pond. With a loading of 2 m2 per population head a primary pond with 750 m2 surface with a depth of 1,2 m will be required, followed by two secondary ponds with 100 m3 content each (5 days detention time in the secondary ponds) and at the end one output equalisation pond with 50 m3 content (approximately 3 days wastewater effluent). The walls and floors of all ponds shall be constructed of selected material which is impermeable after appropriate compaction. Without the consideration of evaporation losses the minimum detention time in the pond system was calculated with 50 days.

The purified effluent from the second secondary pond will be stored in the equalising pond before it will be pumped to the irrigated lands. The equalising pond allows storage of effluent during holidays. The excavation material from the ponds will be used as a protection dam against rain water flooding of the ponds. According to the RSA Water Act 54 (1956) as amended which still is valid in Namibia it will be required to erect a fence around the pond system and the irrigated lands. Green fodder from the lands has to be cut and dried on these lands before consumption. Additional excavation material from the ponds which will not be used in the flood protection dam around the system could be used as a flood protection dam for the community, if required.

According to the author's experience it should be advisable to eliminate any sandfilters as input systems to oxidation ponds. Normally these sandfilters are a source of nuisance like bad odours and mosquito plague. In place of sandfilters a recess of 1,8 m at the beginning of the (normally 1,2 m deep) primary pond should be designed in order to easily collect the sludge concentration at this point.

In some cases it could be worthwhile to investigate the feasibility of aerated lagoons against waste stabilisation ponds where land is restricted and cheap energy is freely available. Whereas stabilisation require 1,0 - 2,8 m2 per person, is the value for aerated lagoons only 0,15 - 0,45 m2 per person (Soli Arceivala, 1988).

All land required for the wastewater purification plant and the irrigated land and any land between should be owned by the concerned community. For the irrigation in sandy soils approximately 7,5 ha (20 m2 per person) are required.

 

2.4.2.2.1 PRIME COSTS

 

(i)    Sewage pipelines to pond system 1.200 m @ R 40
       R 48.000

(ii)   Sewage pipelines to unconnected sewage points 100 m @ R 35
       R 3.500

(iii)  Manholes 12 @ R 2 200 R 26.400

(iv)   Conservancy Tank 2 m deep 84 m3 content (14 x 3 x 2 m)
       R 45.000

(v)    2 Pumps Gormann & Rupp, 1 diesel and 1 electrical
       R 26.000

(vi)   Sewage pond system with flood dams 4.200 m3 @ R 15
       R 63.000

(vii)  Irrigation lands 7 500 m2 @ R 2 R 15.000

(viii) Irrigation pump with power line and irrigation pipeline
       100 m long R 15.000

(ix)   Fencing around ponds and irrigation lands 650 m @ R 20
       R 13.000

(x)    Construction costs escalations, general contingencies and
       professional fees 40% R 101.960

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Total project prime costs : Solution 2 R 356.860

 

2.4.2.2.2 DEPRECIATION AND RUNNING COSTS

 

(i)  2 Sewage and 1 irrigation pump per day R 15

(ii) Labourer 1 per day R 25

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Total per day R 40

Total running costs per year (260 days) R 10.400

The present worth of total net costs in 40 years at 10% p.a. is R 474.812,20 for a salvage value of R 23.000. This solution requires high project costs but small running costs. The total costs per year are more economical than those for solution 1 (R 621.908 for solution 1 against R 474.812). It is also an disadvantage that the wastewater has to be treated in the ponds and irrigation lands which will be outside the grounds of the station because inside there will be no space for it.

 

2.4.2.3 SOLUTION 3 : ROOTZONE PONDS

 

Rootzone wastewater treatment systems are a relatively new development in sewerage purification works. The feasibility proposal for this type of wastewater treatment is based on the findings of Kickuth for wastewater and sludge treatment by means of the rootzone method. The plant should be constructed in such a way that no open wastewater in the contrary to the oxidation pond system will occur. The water will be treated in a closed system under the ground with the resulting prevention of any smell and contamination. The plant should be fitted into the landscape as a biotope and serve as a natural part of the environment. The treated water can be used for crop irrigation for products directly suitable for human consumption.

The annual running expenses should be very low, in the same order than those for open oxidation pond systems. The cost estimate for the rootzone treatment is based on prices for a similar plant construction for the 'E.L.C.I.N.' Medical Mission Station at Onandjokwe/Ovamboland for July/August 1988.

The rootzone basin consists of a 600 mm deep basin which has to be sealed at the bottom and the sides by a sealer in order to prevent percolation into the ground water. The basin is filled with a suitable mould clay. The biological activity depends on the soil components which are important for the function of the plant. Reeds (Phragmites communis) are then planted in the basin. These plants can transport oxygen through their pores down to the roots to provide not only the roots but also the substratum next to the roots with oxygen. Their high oxygen content increases the activity in the soil.

The wastewater goes through an inlet into the rootzone area, flowing horizontally through the basin. During passage through this area the organic compounds and nitrogen is eliminated by means of the micro-organisms in the soil. By this treatment the organic matter is decomposed biologically reducing the nutrients, faeces bacteria and pathogenic germs, as the rootzone plant serves as an anaerobic filter. However, the roots of the reed plants will not be fully grown until two to three years after construction of the system. The purification performance will be at average 50% after the first year and 75% after the second year.

A typical wastewater consumption and composition pattern under Namibian conditions has been reported by 'Namibia Consult Incorporated' (1988) for the proposed Otjovasandu Rest Camp in Etosha [30]:

Wastewater quantities m3/day (Otjovasandu): 110

Wastewater composition:

Total solids 720 mg/l
Total dissolved 500 mg/l
Fixed 300 mg/l
Volatile 200 mg/l
Total suspended 220 mg/l
Fixed 55 mg/l
Volatile 165 mg/l
Settleable solids 10 mg/l
Biochemical oxygen demand, 5-day (B.O.D.5) 220 mg/l
Total organic carbon (T.O.C.) 160 mg/l
Chemical oxygen demand (C.O.D.) 500 mg/l
Nitrogen (total as N) 40 mg/l
Organic 15 mg/l
Free ammonia 25 mg/l
Nitrates 0
Phosphorous (total as P) 8 mg/l
Organic 3 mg/l
Inorganic 5 mg/l
Chlorides 50 mg/l
Alkalinity (as CACO3) 100 mg/l
Grease 100 mg/l

The treatment of this wastewater has to be established according to applicable standards for the disposal of treated sewage effluent which is ruled by a 'general standard' but with certain relaxations as approved by the Government: The following standards are applicable:

pH 5,5 to 9,5
Faecal Coli 0
C.O.D. 75 mg/l
Total dissolved solids max. increase of 500 mg/l
Suspended solids no limits
E Coli 10 000/100 ml

The dimensioning of the rootzone plant in our case study is based on the wastewater quantity of 20 m3 per day, on loading and applicable standards. The special standard being important for complying with the regulations are C.O.D. and Faecal Coli. The C.O.D. content will be reduced from the raw water content of Chemical Oxygen Demand 'C.O.D.' 500 mg/l to 75 mg/l after treatment. 600 m2 basin surface will be required to achieve this standard. The required rootzone volume is approximately 120 m3. The pond volume is 360 m3. The retention time of the wastewater is 6 days in the rootzone.

 

2.4.2.3.1 PRIME COSTS

 

(i)    Sewage pipelines to rootzone plant 1200 m @ R 40
       R 48.000

(ii)   Sewage pipelines to unconnected sewage points 100 m @ R 35
       R 3.500

(iii)  Manholes 12 @ R 2 200 R 26.400

(iv)   Conservancy Tank 2 m deep 84 m3 content (14 x 3 x 2 m)
       R 45.000

(v)    2 Pumps Gormann & Rupp, 1 diesel and 1 electrical
       R 26.000

(vi)   Rootzone pond system: excavation,removal of soil and
       compacting 1.100 m3 @ R 12 R 13.200

(vii)  Inlet and outlet systems incl. gravel R 20.000

(viii) Sealing of plant by bentonite R 4.500

(ix)   Planting of reed R 1.000

(x)    Irrigation lands 7.500 m2 @ R 2 R 15.000

(xi)   Irrigation pump with power line and irrigation pipeline
       100 m long R 15.000

(xii)  Fencing around pond 250 m @ R 20 R 5.000

(xiii) Construction costs escalations,general contingencies
       and professional fees 40% R 89.040

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Total project prime costs : Solution 3 R 311.640

 

2.4.2.3.2 DEPRECIATION AND RUNNING COSTS

 

(i)  2 Sewage and 1 irrigation pumps per day R 15

(ii) Labourer 1 per day R 25

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Total per day R 40

Total running costs per year (260 days) R 10.400

The present worth of total net costs in 40 years at 10% p.a. is R 428.627,80 for a salvage value of R 18.000. This solution requires high project costs but small running costs. The total costs per year are more economical than those for solutions 1 and 2 (R 621.908 and R 474.812 against R 428.628). The construction costs for solution 3 are 13% lower than those for solution 2. It is also an disadvantage that the wastewater has to be treated in the rootzone plant and irrigation lands which will be outside the grounds of the station because inside there will be no space for it. The quality of the treated effluent is, however, superior to the effluent coming from oxidation ponds and can directly be used for crop irrigation without the stringent restrictions of solution 2.

Although the rootzone wastewater management system seems to be a very appropriate solution for Namibian conditions the high quality of the effluent seems to be unwarranted in many cases. Where, therefore, water quality of the effluent is not of prime concern, it should be looked whether other, more economical, wastewater management system could be developed with still acceptable results.

 

2.4.2.4 SOLUTION 4 : INFILTRATION TRENCHES

 

As outlined above it should be strived to keep the wastewater treatment systems simple, planned and designed for informal settlements tackling pertinent local problems on grassroots level. Above three solution have one in common. They are, although relatively simply to construct and to maintain, quite expensive. It will be tried to develop a grassroots level system which will save considerable funds but is still reasonably effective to achieve an acceptable wastewater effluent which causes no unhygienic conditions and no nuisances. Such an effluent should be used for irrigating cactus as green fodder for instance, with some restrictions according to the Water Act 54 (1956) as amended. These restrictions stipulate inter alia that conservancy effluent may not be used for irrigating raw vegetables or salads which are used for human consumption directly. The lands have to be fenced. Groundwater pollution from seepage must always be regarded as a possibility. From tests reported in the literature (Malan, 1962) it would appear that where the seepage source is in the zone of aeration above the groundwater table, the migration of bacterial pollution would be slight (of the order of 6 m and less). The soils profile encountered in the region of Ombalantu and in most areas of Namibia supports this hypothesis. This was also confirmed by information gained from the Namibian Department of Water Affairs.

Solution 4 proposes to minimise the total length of sewerage pipes by reducing the total number of 13 existing conservancy tanks in our case study to three whereby one additional conservancy tank has to be constructed in order to replace one old redundant septic tank which caused many problems in the past. The remainder of the wastewaters will be treated in two existing conservancy tanks. From these three conservancy tanks the wastewaters will be pumped directly to irrigated lands. The sewage from conservancy tank 1 and conservancy tank 2 will be pumped to the existing cactus garden approximately 200 m west of the Cheshire Home and 100 m north of the excavation dam. The effluents from the laundry/clinic tank (conservancy tank 3) will be pumped to a cactus field approximately 75 m southeast from the laundry. The two wastewater irrigated lands are both inside the area of the community.

 

2.4.2.4.1 PRIME COSTS

 

(i)   Sewage pipelines to conservancy tanks 488,25 m @ R 40
      R 19.530

(ii)  Manholes 5 @ R 2 200 R 11.000

(iii) Conservancy Tank 2 m deep 36 m3 content (6 x 3 x 2 m)
      R 20.000

(iv)  3 electrical pumps Gormann & Rupp R 36.000

(v)   Irrigation lands 4 000 m2 @ R 2 R 8.000

(xi)  Irrigation pipelines 364,5 m @ R 32,50 R 11.846

(xii) Fencing around lands 300 m @ R 20 R 6.000

(xiii)Construction costs escalations, general contingencies
      and professional fees 40% R 44.950

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Total project prime costs : Solution 4 R 157.326

 

2.4.2.4.2 DEPRECIATION AND RUNNING COSTS

 

(i)  3 Irrigation pumps day R 15

(ii) Labourer 1 per day R 25

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Total per day R 40

Total running costs per year (260 days) R 10.400

The present worth of total net costs in 40 years at 10% p.a. is R 268.941,40 for a salvage value of R 13.000. This solution requires moderate project costs and small running costs. The total costs per year are more economical than those of all other solutions (R 621.908, R 474.812 and R 428.627 against R 268.941). The construction costs for solution 4 are 50% lower than those for solution 3 and 56% lower than those for solution 2. This most economical solution 4 creates a hygienic and nuisance-free wastewater disposal system which complies to the regulations of the Water Act 54 (1956) as amended. An additional bonus is the fact that the wastewater will be disposed off on the grounds of the community and not outside as demanded by all other solutions.

 

2.4.3 SUMMARY AND CONCLUSION

 

The solution for the wastewater management system 4 (infiltration trenches) has been developed by 'N.C.I.' for small Namibian communities. Although the rootzone wastewater system is regarded as a very worthwhile alternative to any other conventional wastewater disposal system, it is only economical for high requirements regarding wastewater effluent qualities, for instance, if the high-quality water is effectively used for irrigation projects. The system of infiltration trenches can be constructed, operated and maintained on grass roots level. It is, although unconventional and with less stringent quality standards for the treated wastewater effluent, the most economical of all investigated schemes and is as effective, hygienic and nuisance- free as an oxidation pond or rootzone system. This wastewater management system has been discussed with officials of the Namibian Department of Water Affairs. It was found that approval for such a system would be granted if some measures in terms of the Water Act would be observed.

Furthermore, it is obvious that small communities in Namibia will be able to operate and maintain a simple wastewater disposal system like the infiltration trenches system. Project design should ensure that the user community can run their wastewater facilities without financial support from the outside. And this, once more, demonstrates that technology choice and community participation determine project success or failure of a project.

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