WASTEWATER MANAGEMENT - CASE STUDY
Holcim Cement - Factory and Staff houses, Cape Foulwind, Westport

Introduction
Cape Foulwind, an area known for its sensitive coastal ecosystems, is home to more than 100 staff working for Holcim Cement Works, one of the biggest employers on the West Coast. In such sensitive areas, compliance to environmental codes is paramount to sustainable commercial operation. When the company Resource Management Compliance section became aware of a major sewage issue, understandably, they were very concerned. Sewage from the cement factory and associated staff housing was being discharged, via septic tanks, directly into a local stream. This was a historical situation, known by locals but not reported.
Dave Miller, an environmental engineer specialising in domestic sewage systems, was contacted for fast and practical advice on how to remedy the situation with a best practice solution. Dave visited the site, made assessments, and designed a cost effective solution, taking into account the complexities of the site and ensuring the client's environmental responsibilities were met.

Figure 1: Holcim factory with staff houses between the factory and the sea

Situation Description
The following points added to the complexity of the situation:

• It was found that the staff house sewers also collected storm water from down pipes and roadside sumps. One of the greatest threats to the successful treatment of sewage is infiltration of stormwater into the sewer system. As it is relatively expensive to treat domestic sewage, treating additional large volumes of stormwater usually leads to compromised effluent quality and/or cost blowouts.
• The ground water was trapped by a shallow hard pan, found between 2 and 3 metres deep. In winter, this causes the surrounding land to become extremely saturated, so much so that hand pushed lawnmowers become bogged down in the back yard, and cattle stand up to their bellies in mud in their fields.

The Solution
The feasibility study investigated different options including septic tanks with gravity disposal, septic tanks with LPED disposal, individual sewage treatment plants and a community (cluster housing) system.
The storm water infiltration problem was eliminated by separating the storm water system and laying new sewer drains from each house and contributing factory buildings to separate, water tight, interceptor tanks.
The filtered effluent from each interceptor tank is now pumped to a recirculating tank at the treatment plant where it is timer dosed over the textile media in the treatment pods. After treatment the effluent is collected in a treated effluent tank and pumped via Raam irrigation tubing to a disposal field.
Effluent disposal to land presented a challenge due to the clay pan and waterlogged nature of the ground in winter. The usual local farming method in such situations is to 'flip' the land, which in effect is like digging a garden over with a spade. This means using 20 tonne diggers, excavating down to break the hard pan, and reversing (flipping) the ground layers to provide soakage into the permeable layers below.
The disposal area was flipped and the treated effluent is now applied, via Raam irrigation tubing, at a conservative loading rate of 3mm per square metre per day

Figure 2: Staff houses & factory facilities with interceptor tanks pumping to treatment plant and disposal area

System Summary

Comments
The situation at Holcim was typical of many rural communities with small sections, old sewer drains and septic tanks, and inadequate effluent treatment and disposal. The treatment process and solution designed for Holcim, with little modification, can be replicated for communities of up to 20,000 people, cost effectively avoiding threats to public health caused by ineffective domestic sewage treatment. The first step to correct unsanitary sewage systems is to commission Dave Miller to undertake a feasibility report on the options applying to the specific situation.
Dave presented a paper at the NZ Water and Waste National Conference on this project in Christchurch 2004.

Sewage from each house was relaid and connected directly to an interceptor tank (one per house and factory building)