Operational Measures .1 Cleaning and Maintenance

Heavily polluted sewers often have anaerobic wastewater conditions and favour biogenic sulphuric acid corrosion. The most important operational measures for the maintenance of functioning wastewater discharge facilities is therefore cleaning and maintenance. In par-ticular the input of mineral substances into the sewerage system must be kept small, as the mineral components of deposits require very high flow rates in order that they can be flushed out of the sewer bottom. The mineral solids input can be most effectively reduced by frequent monitoring and cleaning of road gullies and dirt traps.

The regular cleaning of hydraulic stress points, such as initial sewer sections or sewers with storage capacity, also brings an improvement to the wastewater situation as the oxy-gen depleting deposits are removed regularly and the flow rate is increased.

In accordance with a German Federal Supreme Court (BGH) judgement, the complete sewer network has to be cleaned at least once a year (BGH Judgement of 11 July 1974 -Ref. No. III ZR 27/72). Continuous inspections should decide locally the necessity and fre-quency of cleaning of road gullies.

With sewers which are difficult to clean with a tendency to become dirty again a continu-ous cleaning process is sensible. With a process using travelling balls (beads) in the flow or gush-flushing, a permanent sedimentation of solids is prevented by their regular application (DINKELACKER, 1987).

If the cleaning possibilities are exhausted or impossible (e.g. in pressure pipelines), and sulphide forms, then the oxygen balance in the wastewater should be improved through suitable measures in order that no biogenic sulphuric acid occurs in the subsequent gravity sewer.

Wastewater systems should not only be systematically and comprehensively investigated but also more intensively and frequently investigated at stress and hazard points to avoid the formation of corrosion. The inspection results should be documented and regulated for all time intervals (date monitoring system).

5.3.2 Measures with the Occurrence of Corrosion

If corrosion damage occurs in existing systems, then subsequent protective measures are to be taken. Corroded sole areas of shafts or sulphur deposits on the shaft walls and pipe crowns, in combination with H2S, smell indicate that the discharge system is subject to an attack of corrosion. First, the cause of the corrosion attack should be determined (e.g. dis-charges containing acids, discharge of organic acids, disdis-charges containing sulphide or biogenic sulphuric acid corrosion as a result of anaerobic wastewater conditions) in order to exclude the corrosion sources by preventing problematic discharges. With biogenic sul-phuric acid corrosion one should, under no circumstances, delay renovationmeasures too long that a prejudicing of the stability of structures has occurred.

The cause of the occurrence of biogenic sulphuric acid corrosion often lies in the planning (comp. Chap. 5.1). A too small a gradient also causes a too small flow rate. Deposits form and there is anaerobic sulphur transfer. At the same time re-aeration only over the surface of the wastewater level is often insufficient. Also long flow paths to the wastewater treat-ment plant or, in particular, the operation of pressure pipelines with too large retention times have a negative effect on the wastewater quality. An improvement can often be achieved through additional or oxygen feed into the pressure pipeline.

5.3.3 Measures in Pump Sumps and Pressure Pipelines

Frequently problems occur at points, for example in pump sumps or in pressure pipelines with too long retention times. Here, the oxygen balance has first to be checked. An oxygen content of 1 mg/l in the wastewater normally suffices to prevent effectively the formation of sulphide. If the mean O2 content is lower, then first consideration is to be given as to whether an improvement through operational measures can be achieved. Individual opera-tional measures are:

− empty pump sumps as much as possible;

− reduction of pump sump volumes by lowering the switch-in point and thus increase the pumping frequency;

− flushing of the pump sump to avoid deposits by circulation of the delivered wastewater at intervals;

− emptying of pressure pipelines at the end of the pumping process (only practical with short pressure pipelines or small diameters).

If these measures cannot be completely or satisfactorily carried out the dosing of various chemicals or a pressure aeration/pure oxygen gassing are to be considered (comp. Chap.

5.2).

6 Bibliography

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