Other Water Management Parameters - SCENARIOS AND WATER POLICIES

5. SCENARIOS AND WATER POLICIES

5.2. Other Water Management Parameters

There are four environmental media receiving waste water discharge in the current version of the model: ground, rivers, lagoon and the ocean beyond the reef. Direct discharges to the rivers and the lagoon, and pollutant transport by the rivers to the lagoon is properly handled by the model. The missing link in the present version of the model is the pollutant transport from groundwater to rivers and the pollutant absorption capacity of the ground water system.

Lacking any data on groundwater movement and quality, it was impossible to construct a meaningful model of these processes. It is evident, however, that the absorption capacity of the groundwater system is limited. Therefore, direct discharge of untreated sewage from domestic and industrial sources to absorption pits must be eventually phased out. The user can model this transition by modifying the disposal matrix that indicates what fraction of the waste water from different sectors is discharged to which receiving media. Even if this redirected discharge is treated, it will pose an additional load to the surface flow or the lagoon model.

When and how fast this transition takes place depends on when and how fast the signs of contaminating groundwater will make it necessary. Three basic scenarios can be proposed as default:

-

transition soon and fast: phasing out groundwater discharge starts in 1995 and will be completed by 2015;

-

transition soon and gradual: phasing out starts in 1995 and completed by 2040;

-

transition later and gradual: phasing out starts in 2020 and groundwater discharge is reduced to 30 per cent of its original value by 2050.

The geographical location of Mauritius would clearly permit the discharge of untreated sewage in the ocean beyond the reef. If the pipes go sufficiently beyond the reef, wastes are diluted by the ocean and are not expected to pose any significant repercussions on either the corals or the lagoon. Yet, this solution does not appear to be environmentally friendly. Moreover, the costs of building and operating these kinds of "disposal facilities"

might well be close to what it would take to build and operate treatment plants.

Currently none of the sewage outlets go beyond the reef. We have no data about the construction and operation costs of such facilities. If this option will be seriously considered in the future, the model can be easily modified to accommodate it.

REFERENCES

CSO. 1988. Digest of Industrial Statktics 1988. Rose Hill: Central Statistical Office.

CWA. 1988. Hydrology Year Book 1984

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¹⁹⁸⁵

-

^1986.Port Louis: GOM Ministry of Energy, Water Resources and Postal Services.

CWA. 1989. Updating of Master Plan for Water Resources. Water Demand. Port Louis:

GOM Ministry of Energy, Water Resources and Postal Services.

CWA. 1991. Master Plan Study on Water Resources of Mauritius. Conclusions and Recommendations. Port Louis: GOM Ministry of Energy, Water Resources and Postal Services.

Falkenmark, M. 1988. Sustainable development as seen from a water perspective. Pages 71-84 in Stockholm Group for Studies on Natural Resource Management (SGN), Perspectives of Sustainable Development. Some Critical Issues Related to the Bmndtland Report. Stockholm: SGN.

GOM. 1991. White Paper on National Environmental Policy. Port Louis: Government of Mauritius.

GOM/World Bank. 1988. Mauritius: Environmental Investment Program for Sustainable Development. Port Louis: Government of Mauritius.

Holm, E. 1992. Land use change on Mauritius. Manuscript. Laxenburg, Austria: ILASA.

U f , G.O.G. and C.H. Hardison. 1966. Storage requirements for water in the United States. Water Resources Research 2(3):323-354.

Lutz, W. and A.B. Wils. 1991. The demographic discontinuities of Mauritius. Pages 39-65 in W. Lutz and F.L. Toth (eds.), Population, Economy, and Environment in Mauritius.

CP-91-01. Laxenburg, Austria: ILASA.

Lutz, W. and F.L Toth, Eds. 1991. Population, Economy, and Environment in Mauritius.

CP-91-01. Laxenburg, Austria: ILASA.

Manrakhan, J. 1991. "Mauritius 2000 and the IIASA research framework: A view from the Reduit Campus. Pages 9-25 in W. Lutz and F.L Toth (eds.), Population, Economy, and Environment in Maunfius. CP-91-01. Laxenburg, Austria: IIASA.

MEQOL 1991. State of the Environment in Muuritius. Port Louis: GOM Ministry of Environment and Quality of Life.

MEPD. 1988. National Developmenr Plan 1988-1990. Port Louis: Ministry of Economic Planning and Development.

MIIT. 1990. Mmuitius at Crossroads: The Industrial Chdlenges Ahead Port Louis: Ministry of Industry and Industrial Technology.

Padya, B.M. 1989. Weather and Climate in Mauritius. Moka: Mahatma Gandhi Institute.

Prinz, C. 1992. Modeling the Population of M-tius. WP-92-43. Laxenburg, Austria:

IIASA.

Rathnam, M.S.V. and K Opsal. 1989. Preparation of an Environmental Action Plan for Mauritius. UNEP Industry and Environment; July-December 1989, pp. 29-32.

Wollman, N. 1960. Water Suppty and Demand Committee Print No. 32. Senate Select Committee on National Water Resources, 86th Congress, 2nd session. Washington, D.C.

Wollman, N. and G.W. Bonem. 1971. The Outlook for Water. Quality, Quantity, and National Growth. Baltimore: The Johns Hopkins Press.

World Bank. 1989. Economic Development with Environmental Management: Strategies for Mauritius. Mimeo. Washington, D.C.: World Bank.

Xenos, C. 1991. Family planning and fertility change in Mauritius. Pages 67-85 in W. Lutz and F.L. Toth (eds.), Population, Economy, and Environment in Mmritius. CP-9 1-01.

Laxenburg, Austria: IIASA.

APPENDIX. TECHNICAL DESCRIPTION OF THE WATER MODEL A l . Water Supply Model

Water supply is calculated in terms of streamflow that can be depended on 95 per cent of the time. The amount of dependable flow can be increased by regulation, that is by constructing water storage facilities to smooth seasonal variations in precipitation and run-off. Quality aspects of water supply are also considered and the flow must satisfy the user-specified quality criteria (see Section A2).

The initial task in the water supply model is to determine the amount of initial flood control storage (IFCS) that is the amount of storage in the base year of the model which was primarily built and operated for flood control purposes. This is required to determine the present total gross storage (FTCS,) in the base year. Data in the table of Past Reservoir Inventory (PRI, see Table Al) are used for these calculations:

IFCS = PRLCAPAC (Afm3)

PRIJURP =FC

PTGS, = (2 P K C A P A C ) - IFCS (Afm3)

Table Al. List of existing reservoirs (as of 1989) (PRI table). Source: CWA 1991.

Water depth

For each time step (every five year period), the model determines the amount of available storage and the resulting minimum sustained flow.

Step 1: Calculate present total gross storage (PTGS) by adding the newly completed storage capacities which depend on the user-specified rate of implementation to the old total storage capacity.

The user is provided with all necessary information about the potential new reservoirs that might be constructed in the future. This information is summarized in the new reservoir inventory (NRI) table in Table A2. When setting a scenario, the user simply specifies the year in the scenario period in which construction of one or more reservoirs should be started. Costs are handled in the economic scenario and the new storage capacity is assumed to be available 5 years later.

PTGS, = PTGS,-, +

C

(NRI-CAPAC) (Mm 3, ( , - I ) < NRI-YOCst

Step 2: Calculate present total available storage (FTAS) by adjusting for the fraction of the initial (base year) flood control storage (IFCS) available for flow regulation in the present year at the current level of regulation. Only part of the IFCS is assumed to be available for flow regulation, although this fraction is assumed to be increasing as the level of regulation is increasing. The actual fraction of IFCS available for flow regulation is the ratio of the present net flow (PNF) in the previous time step to the maximum attainable flow approximated by the mean annual flow (MAF).

MAF and the initial value for PNF are derived from the historical flow data (see Table A3). These data had to be revised upwards in order to include upstream water losses due to imgation which were not captured by historical flow records.

PTAS, = PTGS, - ( 1 - pNFr") IFCS (Mm3) MAF

Step 3: Calculate the present gross flow (PGF) by combining the PTAS with the flow- storage function (FSF). Depending on the user-specified level of dependence (95 or 98 per cent), the model will use the appropriate flow-storage function.

The flow-storage function (see Table A4) contains revised values to account for irrigation losses in the historical observation period.

PGF, = FSF (PTAS,) (m3/see)

3 1

Table A2. List of possible future reservoirs (NRI table). Source: CWA 1991.

Surface

Table A3. Historical flow data. Source: CWA files. francoise constance 0.000 0.000 0.000 rich fund canal f.u.e.1 0.000 0.006 0.016 pierrefonds pierrefonds 0.000 0.000 0.000 tunnel

seche

Table A4. Flow-storage function (95 per cent reliability).

Step 4: Calculate the present net flow (PNF) by adjusting the PGF value for the increasing evaporation losses from the new storage reservoirs. Evaporation losses from existing reservoirs (from those completed before the base year) are already captured by the historical flow data. The evaporation loss rate is calculated by deducting the basic evapotranspiration rate of the vegetation

' cover before the reservoir was constructed from the reservoir evaporation rate.

%MAF

PNF, = PGF,

-

TEVAP,/SECYEAR (m3/sec)

SECYEAR is a constant (31.536*106) to convert annual data to secundum basis.

In order to make it possible for the user to study impacts of exceptional drought periods, the long term PNF, value can be reduced according to a user-specified ratio (assumed reduction inflow--ARP). The actual present net flow (APNF,) in this case will be:

APNF, = PNF,

*

⁽1 - ARP1100)

The APNF value in each time step will be used in the water balance computation to establish whether the amount of APNF is sufficient to satisfy water requirements calculated in the demand part of the water model.

A2. Water Requirements Model

Two components of the water requirements are considered in the Mauritius model:

Losses from withdrawal uses and the dilution flow required to keep the water quality in streams and rivers above the user-specified standard values. The total water requirement is expressed in terms of stream flow:

7WR = TWL + RDF _(m3/sec)

(total water requirement = total withdrawal losses

+

required dilution flow) Withdrawal losss

There are two components of losses associated with water withdrawals for industrial, agricultural, or domestic purposes in Mauritius. First, the net water consumption (NWC), that is the amount of water not returned to the natural watercourse because it was evaporated, transpirated by plants, incorporated into products, etc. Second, the direct discharge (DD), that is the amount of waste water which was discharged into receiving media other than streams and rivers (e.g. lagoons, or deep ocean).

7TVL = NWC + DD (m3/sec)

NWC is derived from the 110 table. Table A5 presents amounts of water delivered to various sectors of the economy to produce 1 Million Rupees worth of output, hereafter called specific gross intake (SGI). The last row shows the amount of water delivered to households per person. Combined with the actual values of output and actual number of people, the total amount of water (gross intake

-

GI) diverted from the natural watercourse, surface or underground, for the economic and domestic sectors can be calculated.

where OUTPUT(j) is:

for j= 1,

...,

15 MRs of output in the given year (110 model)

for j= 16 number of people (total population) in the given year (POP model).

The difference between the amount of water removed from and returned to the natural watercourse is the net consumption. For each (present and future) sector of the economy, a rate of loss (RL) value was estimated that indicates what fraction of the water intake will be "used-up" by that sector:

Table AS. Water use and waste water discharge coefficients.

Note: R L = Rate of loss

SGI = Specific Gross Intake

SGDB = Specific Gross Discharge of BOD

DD accounts for the amount of water which is not returned to freshwater course (streams, lakes, rivers), but rather discharged into brackish or salty waters. In Mauritius, a considerable part of waste water is directly discharged into the lagoons. This water is lost as freshwater, therefore the appropriate flow data must be reduced by:

0')

where DISP is a 4 16 matrix indicating for each sector

0)

the fraction of waste water discharged into the ground (e.g. soak pits; i = 1), into the lagoon (i=2), into the deep ocean (i =3), or into streams/rivers (i =4) (see Table A6).

Table A6. Fraction of waste water discharged into each of the 4 receiving media (DISP).

Dilution flow

RDF is calculated from the amount of pollutants discharged into the streams and rivers after various levels of treatment (pollutant removal) and from the specified water quality standards. Three types of pollutants are considered: Biochemical Oxygen Demand

(BOD), Nitrogen (N), and Phosphorous (P). RDF is calculated for each (QB, QN, QP, respectively) and the largest of the three RDFs becomes the ruling dilution flow:

BOD

Table A5 includes data related to the amount of organic wastes generated and discharged with waste water by each sector. Organic waste load is measured in terms of BOD and it depends on the technological coefficients, the structure of the economy and the level of activity in each sector in the economic model and on the size of population in the demographic model. Numbers in Table A5 indicate for each sector the amount of organic wastes (kg BOD) generated in producing each 1 Million Rupees worth of output. For the domestic sector they show the amount of BOD per person-year. Figures are called specific gross discharge of BOD (SGDB). Gross discharge of BOD (GDB) from each sector is then calculated as follows:

where OUTPUT(j) is:

for j= 1,

...,

15 MRs of output in the given year (I/O model)

for j = 16 number of people (total population) in the given year (POP model).

Only a part of this amount of total generated BOD will reach one or another component of the water system, because part of the waste water will go through treatment processes.

The link between gross discharge figures and the actual net discharges of pollutants was established by introducing a treatment matrix (T) and a treatment efficiency vector.

Elements of the 4 16 matrix T indicate what fraction of waste water is subject. to one of four levels of treatment: i = 1 indicates raw discharge (no treatment); i = 2 means primary treatment (removing a portion of the suspended solids by plain sedimentation, Imhoff tanks, sedimentation lagoons, etc); i = 3 is the fraction undergoing secondary treatment (involving biological processes which satisfy the 0, demand to decompose part of the organic matter in the waste water); and i =4 indicates tertiary treatment (additional polishing by further stabilizing/removing pollutants). Note that BOD discharge from sectors 1 and 2 is negligible, therefore the corresponding elements in all treatment- related arrays will be 0. The 16-sector arrangement was kept for technical reasons.

The four elements of the treatment removal ratio vector (TRRB) show the average efficiency of BOD removal for each of the four treatment levels listed above. For example, TRRB(2) =0.30 means that given the typical primary treatment technologies in a specific year, they provide an average of 30 per cent BOD removal.

The T and TRRB matrices are updated in the model as technologies are developing and new user-specified investments are made in waste water treatment plants. The treatment section of the model consists of two parts. The first part updates the treatment capacity matrix based on investment costs and the user's decision to invest into building new

treatment facilities. The second part is calculating current values of T from the amount of BOD produced and from the currently available capacity at each treatment level.

The first task is to update the treatment capacity matrix (TC) over time. In each time step, the treatment capacity is increased by the amount of new treatment capacity (NTC) completed in the previous period (5 years):

NTC depends on two factors: the cost of building and operating new treatment facilities and how much money the user is allocating for constructing these facilities. Treatment costs are defined to include both construction and operating costs so that elements of the COT(4,16) matrix (see Table A7) indicate the total costs of 1 m3 of waste water treatment capacity to handle sewage from sector j at the given level (primary, secondary, tertiary) of treatment.

Table A7. Cost of treatment ( ~ s / m ~ treatment capacity).

Given the amount of money allocated by the user for investments in tkatment and the COT matrix, the new treatment capacities (NTC) will be (note the 5-year delay to complete construction):

Based on the updated inventory of treatment capacities, we are now in the position to construct the T matrix. Full capacity utilization is assumed when calculating what fraction of waste water is going through each of the four different treatment levels. For each sector (j) of the economy, the amount of treatment at the highest level (tertiary treatment) is calculated first.

For each sector u=3,

...,

^16),the amount of waste water discharge needs to be determined:

Then:

i f X 4 u ) > 0 then: T ( 4 , j ) = TC,(4,j) RETO') eke: T ( 4 J ) = 1 ; T ( 3 j ) = T ( 2 j ) = ~ ( l j ) = 0

In the first case, the full amount of waste water is not treated at the tertiary level, therefore the ratio corresponding to the tertiary treatment capacity will show up in the T matrix. In the second case, there is sufficient capacity to clean all the waste water from the sector at this level, thus other elements in the appropriate column of the T matrix will be zero.

We have the same procedure at the secondary treatment level:

if

^{X 3 u )}^{> 0} then T(3 j ) = Z $ ( 3 J ) RETO') eke T ( 3 j ) = 1 - T ( 4 J ) ; T ( 2 j ) = ~ ( l j ) = O

All waste water which was not cleaned at the tertiary level and exceeded the capacity available at the secondary level, might be subject to primary treatment:

if

^X2Cj)^>⁰ ^then ^T(2,j)⁼

x,

(2 ,j RETO') eke T ( 2 j ) = l - T ( 4 j ) - T ( 3 j ) ; T ( ~ J ) ~ = o

X2(j)>0 for any sector indicates that part of its waste water could not be treated even at the primary level. This part counts as raw discharge:

T ( l j ) = 1

-

T(4 j )

-

T(3 j )

-

T(2,j)

.

Treatment efficiency

Another factor determining the actual pollutant removal from the waste water is the efficiency of treatment ca1,led treatment removal ratio (TRRB) in the model. In order to keep the model simple, we assume that, with proper investments into the maintenance of treatment capacities already existing or created during the model's life-time, there will be a slight improvement in the efficiency of pollutant removal for each type of treatment.

The slow increase in the average efficiency reflects the improvements resulting from the renewed older capacities and the higher efficiency of the newly established plants. The actual rate of improvement is specified by the user in order to make it possible to test different assumptions.

The corresponding equation for updating the TRRB vector will be:

TRRB(i), = l'??RB(i),-,

*

⁽¹⁺^EFFIMP) ⁱ^=2,3,4

where EFFIMP shows the average efficiency improvement (e.g. 0.003 means 0.3 per cent improvement in each five year period).

BOD discharee

The above additions make the treatment part of the water requirements model complete.

The net discharge of BOD from each sector [NDBU)] is now calculated as follows:

Finally, the DISP matrix shows proportions of the various receiving media (ground, lagoons, ocean, streamsllakes) as discussed above. Only the fraction discharged into streamslrivers is considered for calculating the required dilution flow. The net BOD load to the freshwater system is:

NLB = NDBU)

*

DISP(4 J ) (kglyear)

1-1

Our objective is to determine the amount of water that is necessary to dilute the NLB amount of BOD (considering natural regeneration processes) so that the required water quality standard for BOD will be met.

The required dilution flow (QB) is calculated from a simple biodegradation model. The model assumes uniform load and no regeneration which means purification of only one discharge is taken into account:

where

k2 = reaeration reaction coefficient r = reoxygenation efficiency term D = DOS

-

RQSD

DOS = D.O. saturation level at a given temp (Mauritius average) (mg/l) RQSD = required quality standard for dissolved oxygen (mg/l)

--

user-specified Nitrogen

The amount of N discharge from various sectors of the economy is derived from the parameters of BOD discharge. Given the typical technologies in various economic sectors, ratios of BOD to N (CRN) per unit of waste water discharged can be determined. These ratios are listed in the CRN vector for each sector in Table A5.

The gross discharge of N now can be calculated as follows:

Sectors 1 and 2, the two agricultural sectors, represent a special case in this respect.

Except for animal husbandry which is part of Sector 2 and its relative weight cannot be determined from the economic model, there is practically no discharge of BOD from these sectors, but considerable amounts of Nitrogenous fertilizers are washed to the

Im Dokument Models of the Water Systems in Mauritius (Seite 30-46)