Introduction
South Africa has a broad range of industries, including diverse activities such as mining & metals processing, oil & gas, bulk & speciality chemicals, pulp & paper, food & beverage, textiles and pharmaceuticals. Being a developing country, the expansion in some sectors is rapid, presenting serious challenges in terms of protecting our already-stressed water resources. Water is key to the livelihood of South Africa business and hence the correct management of water within our industrial sector is crucial to future healthy economic growth.
This blog deals specifically with wastewater originating from our industries and provides some guidelines on how to manage it. Topics include contaminant types, treatment options, appropriateness of the technology and typical design steps.
Industrial wastewater contaminants
The composition of the wastewater clearly depends heavily on the industry from which it originates. Typical contaminants would fall into one or more of the following categories: dissolved organics, suspended solids, priority pollutants (e.g. phenols), heavy metals, colour, nutrients (nitrogen and phosphorus), oil & grease, refractory compounds, volatiles and aquatic toxicity.
Each of these pollutants, if untreated, has a different effect on the receiving water. Excess organics will lower the dissolved oxygen level of the receiving water, threatening aquatic life and plant growth. Suspended solids will form sediments and scums, which in turn will also deplete oxygen and produce noxious odours. Priority pollutants and metals can cause certain taste and/or odour problems in the water; in some cases they are carcinogenic over a prolonged period. Colour pollution is an aesthetic issue, and nutrients such as N and P will promote eutrophication, stimulating the growth of undesirable algae. Refractory compounds are unbiodegradable, oil & grease result in unsightly scum, volatiles lead to odour and certain contaminants are simply toxic to aquatic life.
Depending on the type of pollutant, the treatment process should be selected accordingly and a certain effluent may require a combination of unit operations to achieve the correct final effluent quality.
Treatment options
Waste minimisation
A great deal of capital expenditure on an end-of-pipe treatment solution can be saved by critically examining the sources of waste within a facility and then trying to minimise their discharge. This can be achieved through improved management of materials and operations (e.g. substituting toxic materials with those less harmful to the environment), modifying equipment (to be more efficient and produce less waste), alter production process (e.g. allowing segregation of polluting and non-polluting streams) and implementing recycling and re-use schemes (e.g. closed loop systems). Not only do such practices lessen the load on the environment, but they also often improve an operation’s profitability through reduced loss of product.
In-plant treatment
Where certain streams within a factory or process are rich in contaminants such as heavy metals, pesticides and other toxic compounds, they must be treated at source, to prevent an inhibitory effect on potential downstream biological stages. Removal of specific pollutants from a concentrated source is also more cost effective and easier than dealing with a larger, more dilute stream.
Heavy metals require some form of oxidation or reduction, followed by a precipitation and separation (filtration or settling) step. Organic chemicals would need to either undergo chemical oxidation, adsorption, ion exchange or reverse osmosis treatment. Volatile organics should be removed either through air or steam stripping. The treated streams can be routed either to final disposal or further treatment.
Primary treatment
Primary treatment removes solids & oils, neutralises excessive acidity or alkalinity and prepares the effluent for either further downstream treatment (biological or chemical) or for final discharge. Unit operations include flow equalisation, acid or alkali dosing, hydrocyclones, static or rotary screens, flotation and flocculation/sedimentation.
In several local food factory effluents it has been observed that up to 50% of the Chemical Oxygen Demand (COD) is particulate in nature and can thus be removed before any further treatment. Another good example is effluent from a local explosives manufacturer, containing nitrocellulose, wood fibre and TiO2: through coagulation and flocculation with Al2SO4, it is possible to remove about 35% of the COD.
Biological (secondary) treatment
If the effluent contains biodegradable organics, a variety of biological unit operations are capable of significantly reducing the Chemical Oxygen Demand (COD). Whilst there is no absolute cut-off, medium to high organic strength effluents (COD nominally above 3000 mg/l) are often most economically dealt with anaerobically, whereas medium to low strength effluents (below 3000 mg/l) are better treated aerobically.
In South Africa, popular aerobic treatment options include aerated lagoon systems (where sufficient land exists), the conventional activated sludge (AS) configuration plus clarifier (with or without anaerobic and anoxic zones for N and P removal), the sequencing batch reactor (SBR) and the membrane bioreactor (MBR). The latter two technologies do not require separate sludge clarifiers and are thus more compact than conventional AS; specifically, the MBR has the potential to produce high quality effluent, suitable for re-use.
Other emerging aerobic technologies for industrial effluent are the moving bed biofilm reactor (MBBR) and the HYBACS process. In the MBBR, a biofilm grows within engineered plastic carriers that are suspended and thoroughly mixed throughout the water phase. The MBBR system is able to withstand high industrial effluent loads and has a small footprint. The HYBACS process is a combination between rotating mesh disc fixed film technology and conventional activated sludge. The result is a works that has a footprint up to 40% less than conventional AS and uses 50% less energy, with full nutrient removal.
Another interesting variation on the conventional AS process for industrial effluent is the PACT configuration (by DuPont and Siemens), where powdered activated carbon is added to the activated sludge system to buffer the biomass against toxic organics and to adsorb certain refractory compounds.
Anaerobic treatment can be effected in lagoons (where space allows) or in purpose-built reactors. For an effluent volume of 1 Ml/d and a COD of 4000 mg/l, an anaerobic system will use 224 kWh/d versus 2240 kWh/d for an aerobic set-up (based on 80% COD removal). Sludge production for the anaerobic option is 96kg/d versus 640kg/d for the aerobic one. The anaerobic system will produce biogas containing about 340 kW of energy. These figures illustrate that where anaerobic treatment is practical, it has distinct advantages over an aerobic equivalent, a difference that becomes more pronounced as the effluent strength increases. It should, however, be appreciated that if the final effluent is to be discharged to a local watercourse, secondary aerobic polishing would be required.
Both low and high rate anaerobic reactor configurations exist, the selection often being made based on the anticipated ease of treatment for a particular effluent, together with its biodegradability.
Tertiary treatment
The most common form of tertiary treatment is filtration to remove remaining suspended matter from upstream biological processes or following coagulation in a physical-chemical treatment. This can be gravity or pressure filtration. Typical granular media are sand, anthracite and other metal oxides. Regular backwashing is an important feature. Microfiltration or ultrafiltration may also be employed for solids removal and may be used as pre-treatment if reverse osmosis is intended.
Another form of tertiary treatment is the removal of colour and residual refractory pollutants by use of ozone or other suitable oxidizing agents. Treatment with granular activated carbon (GAC) may also be necessary. In certain cases, electrodialysis is useful to remove various ionic species.
Some industries with high water usage are targeting zero liquid discharge (ZLD) and to achieve this, a combination of membrane and thermal processes can be used: reverse osmosis to complete the tertiary treatment, with the brine undergoing crystallisation. The crystal slurry is then dewatered in a filter press. The thermal treatment can consist either of a brine concentrator, a crystalliser or a horizontal spray film evaporator.
Prior to discharge to the receiving water body or for re-use, disinfection is often required. This may take the form of simple chlorination, chlorine dioxide, ozonation or the application of ultraviolet radiation.
The importance of appropriate and sustainable technology
South Africa is unique in many ways – we have urban and industrial areas that match numerous places in the Western world, but some of our industry is situated in outlying locations where skills and resources are limited. These challenges demand that when selecting a technology, we give attention to options that are both appropriate and sustainable. Technologies that are applied in our urban environments should be different to those in more remote areas. Issues such as robust design, skills requirements, availability of spares, overall plant ownership & management, legislation and capital and operating costs all need to be taken into account when selecting technology. Where possible, water re-use also warrants consideration.
The final scheme that is developed must match the operator skills that will be available. Since the majority of effluent plants are simply a necessary expense and do not contribute in any way to the overall profitability of the business, they generally receive very little attention from maintenance and operating personnel. It is therefore important that the design ensures they are robust and only require minimal operator intervention. If the correct operator skills are not available for a particular technology, it will be doomed to failure, no matter how effective it might be. As a rule, always endeavour to make the design as simple as possible; should advanced technology really be necessary, ensure that operators receive the correct training. Where it can be afforded, having a professional third party organisation operate the wastewater plant is recommended – most companies prefer their personnel to focus on the core operation of e.g. making potato chips or beer; it is seldom that a company’s own employees will be able to dedicate the necessary attention to an effluent plant, the same way as a professional operating firm will.
Specifically on the topic of sustainability, during the design, always look out for opportunities to make the wastewater plant justifiable in its own right, not only through reduction of local authority charges & fines, but also through the recovery of useful by-products, energy and water.
The design process
The design of a suitable industrial effluent treatment process must follow a methodical procedure to ensure a successful and robust solution. The exercise should start with a thorough characterisation of the effluent, in terms of flow, pollutant types and levels. A representative sampling and testing regime is important to obtain a true picture of the situation.
Once the waste streams have been characterised, the next step is to decide upon the most appropriate route and technology for effluent management. Three aspects need be considered:
- Type of pollutants – are they suspended, dissolved, biodegradable or toxic?
- Final effluent quality – where is the effluent being discharged? Must it meet the General or Special Limits or is it for re-use?
- Cost – before a final process selection, a budget costing should be performed since there are often several routes to achieve the same result; only one is the most cost-effective.
After allowing for all waste minimization and in-plant control opportunities (see above), give consideration to each of the primary, secondary and tertiary treatment options, as well as the interaction between each. Draw up a Process Flow Diagram (PFD), complete with a full mass & energy balance and ratings for each unit operation. More than one final process scheme may appear to be feasible, but use further bench scale tests, piloting, costing and above all, simplicity, to make a final selection.
During selection of the process scheme, designers should engage potential equipment and package vendors at an early stage – their experience in similar applications is invaluable and treatment techniques for industrial wastewater often require specialist know-how and track record.
Conclusion
Every industry has unique features and therefore a unique effluent. A successful treatment scheme starts with a thorough fingerprint of the process and a full characterisation of the wastewater. Always consider waste minimisation before developing an end-of-pipe treatment. Research the best practices for dealing with the various contaminants and perform adequate testing and piloting prior to going to full scale. Make sure operations staff are properly trained in both the running and the monitoring of the treatment plant.
A properly designed and well-run industrial effluent treatment facility is an asset to that business as well as the environment.
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