Notes on Planning .1. Preamble

Corrosion problems can be extensively avoided already with the planning of wastewater systems by observation of the following information. Planning measures are particularly suitable for the hindering of biogenic sulphuric acid corrosion. If, despite all planning pre-cautions, corrosive conditions cannot be excluded then a material resistant against corro-sion is to be selected or non-corrocorro-sion resistant material are to be protected.

5.1.2 Location of Wastewater Treatment Systems

With the planning of central wastewater treatment plants, from the aspect of the sulphide problem, catchment area and location are to be so determined that the wastewater reaches the wastewater treatment plant from the source over the shortest distance and in the quickest possible time. With increasing length of collectors and/or increasing flow times and the operation of pressure pipelines the danger of sulphide problems increases.

5.1.3 Composition of Wastewater

Insofar as wastewater is available at the time of planning, so that its properties can be in-cluded in the planned conditions, several wastewater analyses are to be carried out and the analytical results attached to the request for tenders for the pipes. As parameters the following, for example can be considered: temperature, pH value, settlable solids, Chemi-cal Oxygen Demand (COD), magnesium, ammonium, sulphate, sulphide. The pipe sup-plier is to take on the guaranty for the satisfactory corrosion resistance for the so-described wastewater composition.

5.1.4 Indirect Discharger Operations

The indirect dischargers recorded in a catchment area are to be assessed with regard to the discharge of possible corrosively acting wastewater. Experience shows that one has to reckon with the exceeding of the concentration ranges laid down in the communal drain-age bylaws.

A listing of commercial and industrial branches with possible corrosive wastewaters and their boundary values is contained in ATV Standard ATV-A 115. In addition those opera-tions are to be particularly observed which discharge organic acids with their wastewater;

with cement bonded materials heavy acid corrosion from such wastewater can be caused already with p<H values slightly below 6.0. The connecting sewer is to be made from acid resistant pipes. Materials with sufficient corrosion resistance are to be provisioned for road sewers, until a sufficient dilution has been achieved.

Through the discharge of acidic wastewater there is not only the danger of an acid corro-sion in the bottom of the street sewer but also, with wastewaters containing sulphides there is also the possibility of transferring the whole of the sulphide from the ionogenic form into the undissolved (molecular) form as hydrogen sulphide. Already with a pH value of 6.0, the sulphide exists is almost completely as hydrogen sulphide, which emits from the wastewater into the sewer gas space as volatile hydrogen sulphide gas and, as a result, can lead to biogenic sulphuric acid corrosion.

5.1.5 Drainage Procedures

Precipitation events cause a flushing and dilution of aggressive wastewaters in combined wastewater sewers. With slight partial filling, depositing of solids can extensively in com-bined systems so far as the dry weather channel is not developed.

In comparison with combined sewers, normal sewers have a higher partial filling with dry weather so that, through higher flow rates and bottom drag tension less deposits can form as a source for biogenic sulphuric acid corrosion.

5.1.6 Gravity Pipelines Pipe profile:

In comparison with circular profiles, oval profiles are to be preferred with regard to a re-duced danger of bottom deposit formation. In combined sewers oval profiles have a some 20 % higher bottom drag tension than circular profiles.

External corrosion:

At the planning stage investigations into the soil and groundwater aggressiveness with as-sessment in accordance with DIN 4030 03 DIN 50 929, Pt 3, are to be carried out before laying down the pipe material. The same applies for shafts and other structures.

Flow rate:

The flow rate should always lie above the critical velocity at which depositing of solids oc-curs (critical gradient for deposit-free operation see ATV Standard ATV-A 110, Table 12).

Verification of flow rates are also to be carried out for discharge conditions at night-time and areas of back-up of reductions in cross-section, siphons, etc. If the necessary flow rates for a deposit-free operation cannot be maintained due to the topographical condi-tions, the employment of particularly corrosion protected pipes is recommended. In addi-tion measures can be taken in sewer operaaddi-tion (see Sect. 5.3.1).

With the assessment criteria, in addition to annual costs corrosion measures, odour loads and the safety of operating personnel are also to be assessed.

With regard to sewer cross-sections the employment of non-circular pipes and the ar-rangement of parallel pipelines should be considered. In collectors with large diameters channels with small cross-sections can be used before for initial operational periods. With trunk sewers, in particular with the joining of two collectors, a grit chamber should be con-sidered.

Natural ventilation:

The ventilation of sewers improves the desired aerobic condition in the wastewater. Venti-lation takes place through manhole covers, road gullies and via the roof of extended drop pipes (ventilation pipelines) in houses. In addition, ventilation pipelines at provisional ends to a sewer, at intervals of ca. 25 m between shafts with man-accessible sewer sections, at the start and end of a curved sewer, with connection structures, at the high point of cas-cades, at gate valve installations, etc. are recommended. Ventilation points can be so de-signed that an adapter is installed in the crown of the pipeline, from which a vertical venti-lation pipeline of DN 300 can be led up to the surface of the road, terminating with cover box, covering and natural ventilation.

Drop structures:

With favourable terrain conditions, drop structures are to be preferred due to the consid-erably better oxygen supply in normal gradients. However, with wastewaters containing sulphides one should avoid bed drops due to stripping effects, unless special precautions are taken and odour problems are not to be expected.

Fundamentally turbulence with wastewater loaded with sulphides is to be avoided. With branches the velocity gradient is to be kept as small as possible. The delivery of wastewa-ter from lawastewa-terals into lower main collectors should continue to flow, without dropping; for this appropriate drop structures (e.g. bypass for small flows, chutes, vortex drop shafts, elevator siphons, possibly gate valve regulation) with energy conversion possibilities (e.g.

in corrosion protected whirlpool basins) are to be preferred. Measures for corrosion protec-tion and for the treatment of exhaust gas are to be given particular attenprotec-tion. Pipe sockets planned for latter connection are to closed off water tight in order that no wastewater can occur in them.

Shut-off devices:

Openings for flushing and shut-off devices for pipeline flushing are recommended, at least with larger pipe diameters. Shut-off devices are also useful for regulation of run-off, for

clo-5.1.7 Pump Stations and Cross-sectionally Filled Pipelines Pump sumps:

The development of pump sumps of pump stations (vacuum space) has an influence on wastewater freshness. Sulphide-free wastewater should drop into the pump sump from the inlet sewer, so that the take-up of oxygen is improved. With wastewater containing sul-phides exhalation can be reduced if the inlet joins below the pump shut-off level. The pump sump should be emptied at short intervals, as far as possible completely, so that sewer film on the walls and floor are at least partially aerated. Optimum solution is a large wastewater surface for oxygen transfer with a minimum of sewer film surfaces. The con-tinuous movement in the pump sump or self-cleaning bottom gradient (at least 60 % incli-nation of slope) extensively prevent depositing of solids. See also ATV Standard ATV-A 134).

Selection of cross-section:

Several short consecutive pressure pipelines cause higher annual costs than a long pres-sure pipeline with only one pump station, but can prevent sulphide problems. With large discharge variations, e.g. with combined systems, a pipeline with small diameter should be laid for sewage and a further pipeline with larger diameter should be laid for combined wastewater. In order that the wastewater in the combined wastewater pressure pipeline, with occasional several week long dry periods, does not degrade, this pipeline should empty automatically into the outlet or pump sump with favourable terrain conditions follow-ing completion of delivery. For this the pipeline is to be laid with continuous gradient. In siphons a deposit-free operation is often possible only through the selection of two or more adjacent cross-sections matched to the variations of the amount of the wastewater pro-duced or through the implementation as air cushion siphon.

Flow times:

Pressure pipelines and, if required, siphons, are potential sources for sulphide problems due to a lack of wastewater aeration. So far as they are unavoidable they should be kept as short as possible so that even with night-time flows a flow time of some two hours is not exceeded; otherwise corrosion protective measures and, if necessary, measures for odour reduction are to be provided in the lower subsequent gravity sewers.

Flow rates:

Deposits in pressure pipelines favour the clogging of the pipeline and represent a consid-erable source for biogenic sulphuric acid. To prevent deposits in larger pressure pipelines, flow rates of at least 0.5 m/s, otherwise of at least 1.0 m/s during the delivery phase are to be maintained. With a smaller daily total delivery time or with very long standing periods, an even higher flow rate must be selected as lower limit. By the selection of appropriate flow rates with wall shear stresses of at least 3.8 N/m² with daily peak flows, keeps the sewer film in pressure pipelines very thin, so that their contribution to sulphide formation is heavily reduced (THISTLETHWAYTE, 1979).

Outlet:

The transfer area at the end of the pressure pipeline deserves a particularly careful design.

The cross-section of the outlet of the pressure pipeline should stand completely filled even during pump resting times in order that corrosion cannot occur in the pipe crown of the pressure pipeline (crown height of the pressure pipeline = sole height of the subsequent sewer). Turbulence is to be extensively avoided by appropriate channel design in the outlet shaft.

Prognosis for sulphide formation:

Although it has not been possible to achieve good accuracy for a prognosis of the rate of sulphide formation and/or the attack and destruction rates on cement bonded materials with biogenic sulphuric acid corrosion using the well-known computer models (ATV Stan-dard ATV-A 116; BIELECKI UND SCHRAMMER, 1987; HVITVED-JACOBSEN et al., 1988; POMEROY, 1976; THISTLETHWAYTE, 1979; US EPA, 1985), these models still offer the possibility for estimating the corrosion danger due to biogenic sulphuric acid cor-rosion.

Maintaining wastewater freshness:

To maintain the wastewater fresh, air can be coarsely bubbled into the vacuum chamber; it can be prevented, through suitable design, that gas bubbles reach the pump suction area.

With an artificial aeration air extraction system with off-gas treatment, already determined sulphide can be removed from the airspace of the suction chamber. With the employment of spiral pumps there is an additional oxygen input in the spiral.

Aerobic pressure pipeline operation:

The oxygen dissolved in the wastewater is depleted in the pressure pipeline by the micro-organisms in the sewer film, wastewater and, possibly, in the deposits. The daily oxygen consumption OV can be estimated in accordance with (LOHSE, 1987)

OV = 0.024[π ^. D ^. L(ZSh + d ^. ZAbw/4) - Q24. cO2]

with

OV [kg/d] oxygen consumption in the pressure pipeline D [m] pipe internal diameter

L [m] length of the pressure pipeline

ZSh [g/(m^2. h)] oxygen depletion in the sewer film; can be determined using depletion measurements

ZAbw [g/(m^2. h)] oxygen depletion in the wastewater; can be determined using depletion measurements; mainly at 20° C:

- for 2 hours old wastewater 7 g/(m^3. h)]

- for 10 hours old wastewater 15 g/(m^3. h)]

- for 20 hours old wastewater 17 g/(m^3. h)]

Q24 [m³/h] delivery flow, determined over 24 hours

cO2 [mg/l] oxygen content in the wastewater at the start of the

pipeline

If, for security, a remaining oxygen concentration of 1 mg/l is sought in the outlet area of

OC = 0.024[π ^. D ^. L(ZSh + d ^. ZAbw/4) - Q24(cO2 - 1)]

To avoid an anoxic milieu in the pressure pipeline, there is the dosing of compressed air, bulk oxygen, nitrate or hydrogen peroxide. An alternative to delivery using hydraulic units is pneumatic delivery. Insofar as too long anoxic retention times occur only at night-time, subsequent blowing of compressed air is sufficient.

By creating an aerobic milieu in the pressure pipeline not only are corrosion problems kept under control but also other possible sulphide problems (odour emissions, safety hazards for operational personnel, bulking sludge in the biological treatment stage) are prevented or reduced. Attention is to be paid that, with concurrent injection of compressed air or bulk oxygen, biogenic sulphuric acid corrosion can occur with cement bonded pipe materials (e.g. fibre cement and cement mortar clad cast iron) in gas bubble filled pipe crown areas.

Fittings:

In pressure pipelines, which only run empty over partial stretches, aeration and air removal points are to be installed at the high points (problematic due to the wastewater content substances), at low points drainage devices, and possibly also cut-off facilities. Already with planning the possibilities of chemical dosing and/or gas injection should be planned as a prophylactic measure. With this, additional pressure losses due to gas pockets are to be taken into account in the calculation.

Pressure pipeline gradient:

Gases such as air and pure oxygen can be dosed simply into continuously rising pressure pipelines. Wastewater can be forced out of gently sloped pressure pipelines using com-pressed air; however, a part of the wastewater remains at low points.

Flushing:

With longer pressure pipelines a facility for extraction of water from a receiving water or for groundwater can be provisioned so that the possibility of carrying out flushing in operation exists.

Pressure and vacuum drainage:

Pressure drainage systems are employed in areas which are difficult to drain (wide spread housing), high groundwater levels, slight terrain gradients). In such systems time regu-lated flushing facilities using water or compressed air are used for the limitation of desul-phuration. Vacuum drainage is, in comparison with pressure drainage, less problematic with regard to corrosion susceptibility. See also ATV Standard ATV-A 116).

5.1.8 Soil and Groundwater Conditions

It is to be determined in the planning stage whether the pipelines or structures lie in the groundwater zone; if this applies an investigation of the soil and groundwater for their ag-gressiveness, against cement bonded materials, in accordance with DIN 4030 and, against iron materials, in accordance with DIN 50 929, Pt 3 should always be carried out.