Regional wastewater treatment–an economical solution? Data from 40 years of operation gives the answer

In the 1960-ies most Swedish municipalities discharged raw or insufficiently treated wastewater. Both in the Stockholm region and in the Gothenburg region political discussions concluded in the formation of organisations for regional co-operation for wastewater management. To protect the sensitive local lakes and streams it was judged necessary to transfer the wastewater to the sea. Three organisations, owned by the co-operating municipalities, were formed. The three organisations built regional tunnel system covering areas of over 100 km². Gravity flow tunnels of 55–120 km transfer the wastewater to three central treatment plants. The local sewer system is operated by the municipalities.

The main reason for regional solutions was the possibility to transfer discharges from local inland waters to the sea. There was also at the time a belief that large scale solutions would be more economical (Deininger & Su 1973). The value of a centralised plant versus local plants has later been questioned and another argument against regional solution has been that the heavy initial investments in tunnels will give an unfair distribution of costs between generations. As all the three plants now have been in operation for 40 years or more, it was believed that some of these questions could be clarified by the examination of the economy of the three organisations.

The municipal co-operation is for two of the organisations in the form of private limited companies and the third has a similar arrangement. Therefore all three are called companies below. The companies have to follow the same stringent rules of accounting as public limited companies. This means that the economy is transparent and completely separated from the economy of the owners. The companies produce annual reports. From the reports data on investments, depreciations, interests and operation costs have been collected. The capital costs of the transport systems have been possible to separate from the capital costs of the treatment plants. The operation cost of the tunnel system is more difficult to extract and some assumptions have been necessary to make. The costs for each year have, when relevant, been recalculated to the money value of 2014 by the consumer price index (CPI). All costs are given in SEK. The present relation of SEK to the Euro is 1€ = 9.3 SEK. The total cost is defined as cost invoiced to the connected municipalities minus profit or plus loss from the profit and loss account. Total operation costs are total costs minus interests and depreciations and minus operational incomes. Examples of operational incomes are sale of heat and biogas.

Wastewater Treatment Plant (WWTP) G started operation in 1972 with a connection of 300,000 people that rapidly grew to 530,000 as the tunnel system was extended and then gradually to the present 725,000. Major additions to the tunnel systems were made in 1983, 1989 and 2011. The gravity flow tunnel system now has a length of 130 km. The annual inflow is 110–140 million m³. The plant was operated as a highly loaded activated sludge plant for its first 10 years of operation. Then the plant was extended with primary settlers and more aeration and from 1986 chemicals were added for phosphorus removal. In 1995 the plant was extended for nitrogen removal and has since 2010 a supplementary post denitrification and micro sieves as a tertiary treatment. More details are compiled by Gryaab (2013).

WWTP K started in 1970 with a connection of 250,000 people that gradually grew to the present 490,000. The tunnel system has a length of about 100 km and was complete at the start of operation. The annual inflow to the plant is most years 50 ± 5 million m³. The plant was operated as an activated sludge plant with phosphorus removal by chemical precipitation for the first 28 years and thereafter by biological P-removal supported by chemical precipitation. In 1998 a major extension was made and since then the plant also removes nitrogen and has tertiary filtration. For more information see Käppala (2014).

WWTP S started operation in 1974. Initially the connection was 130,000 that gradually grew to the present 315,000. The tunnel system has a length of 55 km. Also here the tunnel system was almost complete at the start of operation. An addition was made in 1985. At the start the inflow was much lower than the design flow and only half of the treatment lines were operated. The inflow increased and from 1982 all lines were in operation. The inflow has gradually increased from 20 million m³ to 40 million m³ per year. The plant is a moderately loaded activated sludge plant. Since 1991 the plant has operated with nitrogen removal and a tertiary filtration was installed in 1993. In 1997 the nitrogen removal was enhanced by the installation of fluidized beds for post denitrification. For more information see SYVAB (2014).

In Table 1 the percentage of the accumulated costs of capital and operation for transport and treatment are calculated for all years of operation.

Table 1 shows that for the three WWTP's the capital costs dominate for wastewater transport while for wastewater treatment the operation costs are 50–60% of the total costs. For the total costs, treatment stands for three quarters or more. The total costs for the 40 years of operation have also been calculated for 10 year intervals, Table 2.

The transport system operational cost for WWTP S is higher than for the other two plants. This is explained by the fact that the inlet pumps here lift the wastewater over 50 meters, while the lifting heights at the two other plants are <20 meters. For the treatment, the costs decrease during the first 20 years mainly due to increasing connections. Later the cost curves turn upwards due to cost increases for personnel, energy and chemicals for nitrogen removal and for tertiary particle removal to cope with requirements for effluent phosphorus levels below 0.3 mg/l.

The main benefit for all the regional solutions has been the increased quality of the local lakes and watercourses that were relieved of wastewater discharges. From an economical point the transport system is responsible for <20% of the total cost at two of the plants. The somewhat higher cost for transport at treatment plan G is due to a recent expansion of the tunnel system. For wastewater treatment there is economy of scale both in investments and in operations. There is strong evidence that the regional solutions for the three companies have been advantageous from an economical point of view. From Table 2 it is obvious that the benefits of the regional solutions have been more and more pronounced over the years. This is even more accentuated if the fact that the gravity flow tunnels have very large capacities making it possible to accept new connections up to the treatment capacity. When the treatment capacity has to be extended or when the treatment requirements becomes more stringent the regional solutions will have the economy of scale benefit.

If the distribution of costs on different generations is examined, Figure 1 indicates that the first generation takes a larger share due to the initial costs for the transport systems and that also the present generation takes a larger share due to the more stringent effluent requirements. It seem that the generation in between have been favoured.

It could be tempting to make more direct comparisons between the three companies. However this requires a more in-depth analysis as factors as financing costs and depreciation rates differ between the companies.