Alternative technological solutions for the treatment of wastewater from fish processing 

The problem of selecting the appropriate treatment technology for this type of wastewater is determined by the quality criteria of the treated wastewater, but also by the amount of excess sludge produced, which requires further stabilization and management.

Wastewater for testing was collected from one of the largest plants in this industry in Poland.

Table 1. Characteristics of wastewater from fish processing

Pollution indicator	Unit	Value
pH	pH	4,3
COD	mg O2/dm3	33 000
BOD5	mg O2/dm3	15 100
N – Total	mg N/dm3	650
P – Total	mg P/dm3	250
Chlorides	mg Cl/dm3	2 300
Fats	mg/dm3	5 100
Total suspension	mg/dm3	14 500

Wastewater with such characteristics can be treated by two methods:

• Method I – with the use of mechanical wastewater treatment, i.e. elimination of suspensions and fats in the flotation and filtration processes, and the wastewater treated in this way will be further directed to the oxygen reactor and finally treated there.
• Method II – with the use of methane fermentation of raw sewage and further purification in an aerobic reactor

In the first method, in connection with the floated suspensions and fats, it is necessary to introduce a methane fermentation reactor into the technological system, in which the flotates will be fermented together with the excess activated sludge from the aerobic reactor. With a daily amount of 500 m3/d of raw sewage, the amount of suspended solids floated out will be approx. 9,500 kg/d, and the amount of excess activated sludge will be approx. 1,100 kg/d. Full nitrification and denitrification of nitrogen compounds contained in wastewater will be carried out in the oxygen reactor. The low pH value for activated sludge organisms may be a problem.

The amount of heat energy generated from the combustion of the produced biogas is approx. 2 MW/h, while the heat requirement for heating the mixture of sludge for fermentation is about 350 kW/h, which means we obtain 1. 65 MW/h of heat energy for use in the plant.

The amount of fermented sludge is about 1500 kg/d.

In the second method, in which raw sewage is fermented, the amount of thermal energy generated will be approx. 1.6 MW/h, the heat energy demand for heating raw sewage from biogas combustion will be 190 kW/h, the remaining amount of energy will be recovered from digested sewage. The amount of energy to be used for the needs of the plant will be 1.4 MW/h.

The amount of excess sludge is approx. 1200 kg/d, but half of this amount is excess activated sludge, which requires further stabilization. An additional cost in this method is the need to dose the appropriate amount of preparations for the proper conduct of the denitrification process.

As can be seen from the presented possibilities of treating this type of wastewater, each method has advantages and disadvantages. For this reason, the choice depends mainly on the needs of the investor, and in the case of designing an industrial wastewater treatment plant, the issue should always be considered individually.