Waste water treatment

The chemical industry and most other industrial sectors resort to end-of-pipe treatment techniques to reduce waste water and the pollutants it carries. They encompass pretreatment at the source or in combined streams as well as final treatment of collected waste water before discharge into a receiving water body. The main waste water end-of-pipe treatment techniques and their applicability to control the major contaminants in the chemical industry are shown in Table 1.1. However, it is only indicative and the reader is encouraged to refer to the details given in the specific sections of this document indicated in the table.

Table 1.1: Major waste water contaminants and their respective treatment techniques

metals Sulphides Sulphate Phenols Oil Acids, alkalis

Section in this document

Neutralisation (X) (X) X 3.3.2.3.2

Grit separation X 3.3.2.3.3.2

Coagulation/flocculation X X (^{b, q}) X X X (^b) 3.3.2.3.3.3

Hydrocyclone X 3.3.2.3.3.9

Electrocoagulation X X X 3.3.2.3.3.10

Electrodialysis X 3.3.2.3.4.8

Electrolysis 3.3.2.3.4.9

Adsorption X (^v) X X X X X X X 3.3.2.3.4.10

Ion exchange (X) (^d) X X 3.3.2.3.4.11

Extraction X X X X 3.3.2.3.4.12

Pertraction X X X X 3.3.2.3.4.13

Distillation/rectification X X X 3.3.2.3.4.14

Evaporation (^w) (X) (^e) X X X X X 3.3.2.3.4.15

Pervaporation X (^f) X (^f) X (^f) 3.3.2.3.4.16

Stripping (X) (^f) X X X X X (^p) X 3.3.2.3.4.17

Waste water incineration (FT)

(^w) X X (X) (^g) X X (X) (^k) X X 3.3.2.3.4.18

Technique TSS

BOD COD TOC

Refractory COD/TOC

AOX EOX

N total

NH₄-N

(NH₃) PO₄-P Heavy

metals Sulphides Sulphate Phenols Oil Acids, alkalis

Section in this document Biological removal of

sulphur compounds/heavy

metals X X X 3.3.2.3.5.3

Aerobic treatment X (X) (^h) X X X (X) X X 3.3.2.3.5.4

Nitrification/denitrification X X 3.3.2.3.5.5

Enhanced biological

phosphorus removal X 3.3.2.3.5.6

Phosphorus removal by

chemical precipitation X 3.3.2.3.5.7

Retention ponds X X 3.3.2.3.6.2

Sand filters X 3.3.2.3.6.3

(^a) Only solid.(^b) Undissolved organic content.

(^c) Finely dispersed and low concentration. (^d) Ionic organic species.

(^e) Non-volatile organic content.(^f) Volatile organic content.

(^g) Special incinerator equipment required.(^h) Only biodegradable part.

(^j) Undissolved heavy metal compounds.(^k) Transferred to ash or waste water originating from incinerator.

(^l) In combination with sulphate precipitated as sulphides.(^m) Transferred to sludge.

(ⁿ) Colloids.(^o) Ammonia.

(^p) Hydrogen sulphide.(^q) Some macromolecules.

(^r) Side effect of ammonia or nitrate removal.(^s) Side solubilisation.

(^t) Cr(VI).(^u) Includes nitrification/denitrification and one-step nitrogen removal process of Annamox type.

(^v) Including colour agents, surfactants, nitrocompounds, chlorocompounds, phenols. (^w) Techniques applicable on concentrated effluents [ 148, Degrémont SUEZ 2007 ].

NB: (FT) = used as a final treatment technique; (pre) = used in particular as a pretreatment, for example before final biological treatment; X = primary application; (X) = secondary application.

Source: [ 227, CWW TWG 2009 ]

Complex chemical production sites normally have an extensive system for the collection and treatment of process water. There are several approaches to waste water treatment, each of them with its advantages and disadvantages, depending on the situation, including:

 decentralised waste water treatment facilities, treating the aqueous effluent at the source and discharging into a receiving water body (i.e. no central waste water treatment facility on site);

 centralised waste water treatment, normally using a central waste water treatment plant (WWTP);

 central WWTP, with upstream tributary stream pretreatment at the source or in combined streams;

 waste water discharge into a municipal WWTP;

 waste water discharge into a municipal WWTP with on-site pretreatment at the source.

The last two bullet points are special situations of the two preceding bullet points respectively.

The advantages of decentralised waste water treatment or treatment at the source (or the disadvantages of centralised waste water treatment) include:

 the operators of the production installations show a more responsible attitude with respect to the effluent when they are made directly responsible for the quality of their own waste water discharge;

 more flexibility exists for works' enlargement or for reacting to changing conditions;

 facilities for treatment at the source are tailor-made and thus normally show better performance levels (however, the initial performance might deteriorate, e.g. when new installations/equipment are put in place or modifications are carried out);

 in contrast to the centralised biological treatments, there is no (or less) excess activated sludge to dispose of;

 the treatment performance of non-biological techniques is independent of the biodegradability of the waste water streams;

 dilution by mixing of different waste water streams can be avoided, normally resulting in a higher treatment efficiency;

 the cost-benefit ratio can be much better in tributary stream treatment than in centralised treatment.

Decentralised waste water treatment is generally the preferred option when tributary waste water streams with completely different properties are expected.

The main advantages of using a centralised WWTP (or the disadvantages of decentralised treatment facilities) include:

 making use of synergistic effects of mixed biodegradable waste water, i.e. effects that enable microbiological degradation of special contaminants in a mixture with others (or even in a dilution with other waste water streams) whereas the tributary stream alone has poor biodegradability;

 making use of mixing effects, such as temperature or pH adjustment;

 more effective use of chemicals (e.g. nutrients) and equipment, thus decreasing relative operating costs.

There are instances where waste water from chemical industry sites is also treated together with municipal waste water, either in municipal WWTPs or in specially built plants for the combined treatment of municipal and industrial waste water. The joint treatment is frequently arranged in such a way that, because of its high initial organic loading and the tendency for decreasing degradation rates in diluted waste water, the industrial waste water initially undergoes a high

Experience has shown that the joint treatment of municipal and chemical industry waste water generally has neither synergistic nor antagonistic effects on the receiving water, at least as a first approximation [ 22, BMU/LAWA 2000 ] (a contrary example of a coordinated operation of a chemical and a municipal WWTP is described in Section 7.1, Annex I). The pollutant loads disposed of are generally additive.

Advantages of joint waste water treatment may include [ 22, BMU/LAWA 2000 ]:

 the operational stability of joint biological treatment which can be favourably influenced by:

◦ improving the nutrient conditions;

◦ optimising the waste water temperature and thus the degradation kinetics;

◦ equalising the feed volume and load (see Section 3.3.2.1), as long as the daily progress lines of the two waste water streams are correspondingly structured, or can be matched to one another;

◦ suppressing the toxic and inhibitory effects of waste water constituents by lowering the concentrations below the critical thresholds;

 the joint treatment of waste water and excess activated sludge which can, in individual cases, realise savings in investment and operating costs.

Disadvantages of joint waste water treatment may include the items listed below.

 Reduced cleaning performance due to production-related operation disturbances, which leads to increased water pollution because of insufficient treatment of both municipal and industrial waste water streams [ 22, BMU/LAWA 2000 ].

 Quite a number of chemicals can, even at lower concentrations, hinder nitrification. If the nitrification step collapses, it might take several weeks to recover and ensure sufficient nitrogen elimination again. So, to minimise the risk for joint waste water treatment, it is crucial to study and monitor the waste water streams coming from the industrial part carefully for any inhibiting or disturbing factors [ 22, BMU/LAWA 2000 ].

 Combined treatment of waste water streams from different origins bears the risk that persistent contaminants, such as heavy metals and non-biodegradable compounds, can escape control, and sometimes even detection, because of dilution. These contaminants are discharged without degradation into a receiving water body, adsorbed onto the activated sludge and/or stripped into the atmosphere during aeration. This would counteract the obligation to prevent or control these substances at the source. This disadvantage affects all treatment actions on combined waste water streams.

 Combined treatment may result in sludge that is too contaminated for further usage or further treatment, e.g. by anaerobic digestion.

The joint treatment of municipal and chemical industry waste water necessitates appropriate buffer tanks to cope with excess rain in the event of heavy rainfall in order to avoid hydraulic overload, which might lead to increased pollutant discharge accompanied by loss of bacteria from the activated sludge compartment of the WWTP [ 22, BMU/LAWA 2000 ].

Another important aspect of the waste water system is the handling of uncontaminated rainwater and cooling water. In a number of older chemical sites in Europe, only one sewer system is present and rainwater, rinsing water, cooling water and process water are collected in this system and directed to the waste water treatment facilities. Especially during periods of heavy rainfall, this might lead to upsets of the WWTP and lead to increased discharges. Advanced chemical sites have a separate sewer system for the collection of uncontaminated rainwater and cooling water. Details are given in Section 3.3.2.3.6.